u wont go wrong with either.
66u, you will have amazing merlin,
65u, you will get improvements by padavan.
dual cpu on 65u for more processing power for better hw nat and opt/ent support.
external antenna on 66u give u more range, more fw choice like merlin,dd-wrt and tomato.
if u can wait, maybe u'll like ac56 or ac67.
also, if im not wrong, 65u is a 2-stream while 66u comes with 3-stream.