I think you will break more birds with the 12ga (kind of a mathematical no brainer really). As you be more proficient with your 20ga, the difference between it and a 12ga would start to get much smaller.
Skeet is my game, and I have not shot 12ga in years. For me 20ga is plenty. If I move to a game that isn't my specialty (doubles fricking trap comes to mind) then I would like to have that extra lead to put down range. Experience has show me that in these situations I score better with a 12ga (longer shots, and other target presentations that I don't often see).
Which ever gun you buy, just make sure you can get ammo. Generally 12 and 20ga ammo is priced similarly, but not always. 12ga ammo is usually much more available than 20. From there I'd see if I got a better deal on the 12ga gun I wanted or the 20ga gun. I don't personally worry about gun fit off the shelf as I full well expect to go see a gun fitter (or suitably modify the gun for fit myself).
YMMV