On a wall to try?? ...exactly the problem. That someone actually has a dealer that has one Taylor 12-er, let alone more than one, is a find.
I agree with Dennis that 12s in general are chimey/jangly, and of course, Taylors even more so. Given I prefer Taylors and their playability (especially when it comes to a 12, man what a difference!), I'd definitely go for a GS or Jumbo bod. Wood choice? Depends. I owned a 655 (jumbo) and currently own a GS-sized 12, and can definitely tell you they are both more "substantial" in bottom end and "punch" than the GA-size I had played. Now perhaps the wood played a role there. Maple is great: fast attack for good punchy tone, and faster decay so all those strings jangling together somehow sound "clearer" ...folks say fewer overtones with maple, to which I'd have to say is a benefit with a 12; and certainly a benefit to a stage/amp'd guitar. But one can't deny the role a large body plays in projecting a larger voice with bigger lows ...which is particularly "better" given you've got all those additional treble strings occupying more of your sonic footprint!
But if you want a more "harmonically complex" 12er, then mahog or rw would likely suit you better ...but again, in the larger GS/jumbo bods would be my personal preference.
BTW, there's something just so uber cool looking about a jumbo 12, and having owned one, I totally dug it. But in terms of comfort, I have to give the nod to the GS.
Edward