It took me years to appreciate Tusk and fall in love with it, and it gets my vote, hands down.
Rumours is indeed an album full of Mac classics and staples, but I think that's what makes Tusk so much more interesting -- it presents the nuances and less straightforward sides of the band. It's not the basics that every fan should know; it's the subtleties of each songwriter (and they all have wonderful tracks here) that you can sink your teeth into over and over again, each time finding something new.
On another note -- I don't really ever listen to the album Rumours. Having listened to so many live versions of most of these songs, the studio versions sound too polished and, frankly, odd to my ear. Even lifeless, if I'm allowed say that
Not to say that detracts from the album itself, it just makes it less appealing that Tusk to sit down and listen to all the way through.