Top loading all the way here, I've got two top-loaders and I wouldn't go back to purely front loaders if you paid me. Gone are the days of trying to get a reluctant claw-brandishing cat into the carrier which was like trying to thread a disgruntled elephant backwards through the eye of a needle. IMO always get the biggest one that you can as cats like Alfie will splay out their legs like Garfleld on the car windscreen to frustrate top entry in my smaller carrier but the big one is very good for those situations.
Cheers
Leigh