Personally, i think you should get some and/or actually all of the Bob Welch era stuff. I rarely listen to Behind the Mask. I bought it when it came out and i went to the concerts and all but i find the Welch era a more interesting "link in the chain" if you will.
I actually loved "In the Back of My Mind". I also loved "Behind the Mask" and "The Second Time". "Save Me" was pretty good too. I think another one was "Hard Feelings" that i uses to listen to too...Oh wait, then there was "Love is Dangerous" also.
Anyway, I think the Welch era stuff seems to fit better in with FM's roots....I also think this era saw some of Christine's best material.
So, that's my opinion!
