The age old discussion/argument on how the 5dr is more convenient rages on, but I think the 3dr is actually easier when it comes to "installing" the little ones into their Recaro Junior Sport seats

Take my 3dr mk4. I pop the seat forward and enter the car with him on my hip, then when leading forward - pop the young 'un in. When removing him, he leans forward once his harness is off and lifts straight out.
Compared to the Mrs's 5dr Fiesta. I open the rear door with the boy on my hip. I then have to swing him around into the kind of carrying position you would take your new bride up to bed in, to drop him into his seat sideways. When removing, his lean forward makes very little difference as again I have to somehow scoop him up by legs and shoulders in order to lift him up and finally out of the car.
I truly think the 3dr is easier to get the kid in and out. Also pulling the harness tensioner from the front is a lot easier than sideways.