Both have rollers. I have the older 771 model, w/ the open front. its very secure and at the same time very easy to pull the next mag out for reloads.
the model 773 is an improved version. the 773 keeps the bullets inside the mag from coming out preventing a botched reload or worse, a misfeed. it also has a lower profile design for faster clearing of the mag lip.
if i'll get another set, again, i'll order the 773's.