Leaving aside the health risks (As per Wikipedia, "Some argue that situps can be dangerous due to high compressive lumbar load"), is there any benefit to doing crunches as opposed to sit-ups?
From Wikipedia description sounds like sit-ups excercise more muscles than sit-ups.
If specifics matter: Person is a beginner, with no history of extensive fitness or training. Age 35, with slight amount of abdominal fat and 22.8 BMI. No history of back pain but slight scoliosis.
Excercise goal: overall fitness, increase abdominal muscle to look better, anecdotal advice that improved abdominal muscle tone helps improve posture.
 - Yes I'm aware that fat burning through calorie deficit is ALSO required as far as getting anything approximating a six-pack. But obviously developing abdominal muscles is needed just as much as fat burning