depends on what you're trying to achieve. as a general rule, I find that crunches are ideal for just isolating the abs while situps are a more compound motion, involving the hips and back along with the abs. if you're just trying to tone up your 6 pack, your best bet is probably to go with crunches as indicated
here and
here.
however, even more important than just crunch vs. situp is to do a variety of ab exercises. doing several different ones - especially with resistance - prevents the exercise from getting boring and assures that you hit every part of your core. here's a good
list of exercises. I'm a fan of kneeling cable crunches, weighted ball crunches, and leg raises. Don't neglect your obliques either - side bends and twist crunches work pretty well.
EDIT ---> forgot to mention one other important thing. you can do crunches all day long and still not have a six pack if your body fat percentage is too high. to get a 6 pack to really show up, you either need stellar genes or you have to do plenty of cardio/HIIT and be very careful with your diet.