I handled the round butt a few times but never shot one. When I handled the round and non-round side by side, the non-round felt better. To my hand the rounded butt felt like something was missing. The round butt did look really nice though. This is so subjective. Each person will feel something different. Kinda stinks you can't handle some before deciding.
The round butt wasn't any thinner on the models I handled. Not sure if that's an option or not.
EDIT -- I did some surfing around quick and found pics that show the round butt is indeed a bit thinner. Hmmm...I never noticed. Maybe that's what I felt missing??
I'm thinking along the same lines as you about the 3 1/2". They look really nice but the extra step for disassembly (along with balance and slight gain in sight radius) has me thinking 4 1/4".