I'd stick with the T64s. The T74s are better on paper but are heavier. Weight always becomes an issue in robots and the T64s will be more than enough to drive up any other machine and keep going. I ran mine on 32v (8s lipo) and they were too powerful. Had to derate them on the transmitter.

Can't comment on the ampflow as I haven't used them.

Dual channel means it will run two different motors independently. Single channel means it will run a single motor. If you want to have drive you need a minimum of two motors for tank skid steer style control. There isn't really a huge advantage for one method over another.