crunches or regular situps...which is better?

i disaggree with the guy above, they are both beneficial in chiselling your upper abs as well entire rectus abdominus, for more challenging ab workout do crunches and sit-up in 30 degree bend in order to give full contraction to your upper abs...

Both aren't as beneficial because we tend to have a hunched posture, which causes massive compensations in the body. Sit-ups do tend to activate the core, but having something, which increases your range of motion (meaning your body starts lower then flat and has to flex harder to get passed the middle and to the top), you will increase the toning effect of either exercise. Sit-ups or crunches should also be accompanied with nearly double the low back exercises and to also hit the surrounding core area for better development and better tone, oblique situps (diagonally sitting up) are much better overall. You can also try something called Janda Situps, which supposedly shut off muscles that aid during a "normal" situp and thus with no extra muscles working make the exercise much harder.