After getting my knee smacked four times on ORD-IND in 3C, I think the extra legroom it has over 3D isn't quite worth it. 3D would have had more than enough legroom for me - the guy in 3D was 6'6" with really long legs and he was fully extended and fine. 3D also has a place to put your bag under the seat, which is important because 3AC has no overhead bin space directly above.

I'm going to book 3D a couple of times and see how that goes.