I wouldn't say olympic squats are harder. I've done both and although it's difficult at the bottom range of an olympic style squat, it feels too light for me at the top, whereas with a low bar, medium pl style squat it feels heavy all the way and I take mine too legit parallel. Plus, you can use more weight with pl style squats and that right there doesn't make them any easier than a lighter weight full squat imo.
Different strokes for different folks. If you want pure quad developement then I'd go with full squats, but if you want pure strength I'd go with pl style.
As for those that say parallel or slightly under doesn't build the hams and glutes as much, that's just bunk. A pl squat you sit back and down which automatically brings the glutes and ham's into the movement alot. Olympic squats go straight down basically and stress the hams and glutes when powering out of the hole, but imo less than pl style because with a pl style the tension is maintained on the glutes and hams more I think.