There can be only one
 

There is an honest and genuine debate to be had;

Would Lindsey joining Mick, John and Chris (without Stevie) been more successful than Stevie joining Mick, John and Chris (without Lindsey) in 1975 or vice-versa? Obviously that assumes at least somebody else joins with Stevie (perhaps BW stays but LB doesn't join).
Who had the biggest impact???

The truth is, probably neither band would have sufficiently made enough waves. But of the two, who would have garnered more success?

I genuinely don't know. Depends on the other guitarist I guess.

It’s conjecture, but I imagine FM would have continued to do moderately well to even a little better with just LB and CM fronting the band. But for how long? Two or three years tops?

SN would not fit in with the core trio unless LB was there. She ended up being the reason FM became superstars. Without her, they were merely a very fine rock band. But LB’s presence as the bridge between SN and the former blues players was essential in order for it to happen.

In short, FM could only be the iconic band they became with all five of them on board. After LB left in 87, they declined. After SN and CM stopped touring and SN left in 1990, they declined further. After CM outright left in 1998, they were able to continue but without as huge a fanfare as before. After LB was fired in 2018, they repeated themselves as a covers band.

Probably with Stevie the band could get a bigger commercial success than the previous albums, but only if they get also a very good producer; I deduce it from the weak success of Behind the Mask, though it's just a clue, not an evidence.

With just Lindsey, the critics would have said they had very good albums, probably not great commercial successes.

Anyway, in both cases there wouldn't have been the albums we got. There wouldn't have been a Rumours phenomenom.

Stevie was such a major draw as part of the live band. She was the undoubted star almost overnight and the main reason the line-up gained traction. She also brought Rhiannon to the table. Would her songs have worked on record without Lindsey bringing them to fruition?
However, Lindsey was such a phenomenal musician. His musicianship was a significant step ahead of what Bob Welch could give them.

Its obviously the combination of the two that gave people the interest. However, I'm going to stick my neck out and say if only one of them had joined it would have been a band featuring Lindsey that would have found more success. He gave them leadership and I think his nous would have found a way to some sort of success irrespective of whether Stevie was part of it or not. Stevie on the other hand was much more naive regarding the business and music in general. Without a musical leader they'd have remained lost.

Didn't Iovine say he knew Stevie needed someone (that someone was Lindsey) to turn her work into songs to include on albums?

She also never took the initiative since 1981 to become a guitar player. Even Rod Stewart, a true frontperson, plays guitar onstage for Dirty Old Town.

The problem with your scenario is Lindsey actually produced Fleetwood Mac. He made them stars. He took crappy demos like Gypsy, Dreams, Sara and turned them into hits. It’s not debatable who is the bigger talent. Stevie got to work with great producers because Lindsey made her star.

Her stardom got into the bands way. She made the band toxic. He would of never left in 1987 if it wasn’t for her.

By December 31, 1974, Keith Olsen was under contract to make the new Fleetwood Mac album.

Hypothetically, Mick tells Keith to get Lindsey and Stevie, but Lindsey refuses....

“Lindsey doesn’t want in? Okay, well there are these cats named Warren and Waddy you could consider...”

A couple weeks later: “Hey, you’re Christine, right? I’m Stevie. Wanna get some margaritas?“

I dont think adding just Stevie in the mix in 1974 would have changed the band so much. Lindsey changed it with guitar and new production sound. The band probably would have had some success with Lindsey.
But the couple needed each other and the band also needed them and the couple needed the band.
Adding both was like a chemical equation. Each is not as powerful without the other. So just adding one to Fleetwood Mac would not change very much. However Lindsey is a musician and I think they would have had some success with him. The star power of both completely changed the band almost like a new band.
I heard Stevie say once that "Lindsey and her did not get people falling out of their chairs during the Buck/Nicks era until they joined Fleetwood Mac." She sounded a bit resentful because she explained how hard they worked.
The 5 together was like the perfect recipe. You cant take one away during the foundation years and expect the same success.

This is a tricky one, good question.
Well I think Lindsey added the most artistically, but Stevie made the most money for them.
Without Lindsey they would be just an average middle of the road rock band....long finished by now.
Without Stevie they wouldn’t be multi millionaires..
So it’s Art over Cash..
with both of them added, you get the full package..

Stevie attracted people to the band. She attracted me. But without Lindsey, the songs that pull everyone in wouldn’t have existed in the form that we know them. Without Stevie, we would still have OMH and SYLM and GYOW. But without Lindsey we aren’t going to get the same kind of Stevie hits. Furthermore, without the work that Lindsey did on her songs which made her prominent, I don’t think you’re going to get the other producers interested enough to work with her.

Waddy said he thought that Lindsey was wasting his time working on her flower and the vine stuff. He didn’t see the diamond in the rough. Lindsey saw it, polished up the diamond and then everyone wanted to work with her.

Stevie had a very unique voice and people would be attracted to that. But the initial songs that helped her create her aura needed Lindsey. So, I don’t see Fleetwood Mac achieving as much success without Lindsey and just Stevie.

On second thoughts and after reading some info, Bella Donna was N.8 in Billboard 1982 year end chart (got N.1 in a weekly chart) and sold 10 million copies. Tusk sold 4 million copies worldwide. I read Mirage sold 5 million copies.

So, definitely without Lindsey there would not have been the Rumours phenomenon and I've always said that the 5 members lineup was the definitive Fleetwood Mac to me. But about this debate, I think, without Lindsey who knows how would Christine and Stevie songs have sounded. Wth a good producer (not Greg Ladanyi), and probably a third songwriter (the guitarist) the band could have achieved a decent success.

“Successful” means “make more money”? It’s hard to say what would have happened in 1975, when Stevie’s only mystique was being a cute babe who sang funny. She only later developed that aura of being a great songwriter and rock Romanticist, as we all know. But I guess Stevie needed to be in the band for it to make the most money. She developed the quality that turns on rock audiences — that makes them want to watch her. She developed that appeal a lot quicker than Lindsey did, and she had more of it. The mass audience didn’t know that he crafted the arrangements; for all they knew, Stevie did that, or they didn’t think about who at all. By the time Lindsey got a widespread response of any kind from Mac fans, it was almost entirely a specialized, cerebral response: his inventive guitar work, for example, wasn’t going to blow Alvin Lee away, but it was going to add texture and mood to a song so that you could listen to that song a thousand times and a year later you still wouldn’t be sick of it (hugely important to Fleetwood Mac’s commercial success). That’s just not the kind of thing that sells lots of albums or sells out arenas by itself. It needs to complement something. Naturally, by 1977 (not 1975) Stevie had the crossover appeal.

Where did you read 10 million? The only stat I can find is 4 million in the U.S., per Google. Google won't give me worldwide info.

It’s 4 million. He doesn’t know what he is talking about. Tango is 10 million.

Stevie was NOT seen in the early days of FM the way she is seen now. She was openly mocked by most of the men in radio... you used to be able to find links on youtube to vintage recordings of radio DJs mocking how she sang, the infamous goat comparisons, etc. She was NOT taken seriously at all. And she wasn't the performer the world came to know and love back then either.. Ray Lindsey and some others tell some great stories about those days in a recorded interview that's somewhere on youtube about how long it took her to find her footing and that her singing on that first short Mac tour was NOT good. So, without LB to polish up her songs for radio, and with her stage persona not yet solidified (the Rhiannon outfit not yet created, no top hat, etc) I would doubt that she would have set the world on fire in the way that she did. I think they'd be more successful with just LB and his contributions to the recorded music, which would get them more airplay (which was crucial back then). Stevie, in her still developing state in 1975 would not have gained them much at all.