I wouldn't stress the rep scheme too hard. If you feel your working the muscle better with heavier weight and not as strict form then that's what you need to go with or if the lighter weight is working it better, go with that.
Some people stress form too much. Bad form is form that puts you at risk for injury. Otherwise, you know what muscles you're trying to work, so who's to tell you that your form is wrong if you're feeling it the most in those muscles with the way you're doing it?