I guess just the mechanics of it, and the fact that it's easier to change a pin than load plates.
Personally, I say do free weight squats and forget all of the damn machines!
Hack squats cause more direct work from the quads, and of course they take pressure off of the rest of the body compared to free weight squats. I do them once in awhile, but honestly the leg press and hack squat both cause me knee pain, where as a free weight squat does not.
on my leg days, i usually follow the following routine with a little bit of variations.. can you please comment..
seated plate loaded squat machine,
I do free weights for my upper body, but free weights for my lower body is something i haven't considered yet. can you comment on my leg workout and also give me tips on which of the above machines should i replace with free weights..