Crunches places more stress on the abs, meaning the muscles are being used more, if you do the same number of crunches as situps then the crunches will hurt more, but do more in terms of pulling you closer to that goal of a six pack...
Crunches Sit-Ups Leg-Lifts
crunches and sit ups
Crunches and sit ups do not cause the uterus to drop or fall directly. This may happen if weight is lost.
sit ups, push ups, crunches, and military press
There are various types of sit-ups, including traditional sit-ups, crunches, reverse crunches, and bicycle crunches. Each type targets different muscle groups to varying degrees. Traditional sit-ups primarily work the rectus abdominis, while crunches focus more on the upper abs. Reverse crunches target the lower abs, and bicycle crunches engage both the upper and lower abs along with the obliques.
Crunches build muscles while situps build abdominal muscles. Each workout is important based on individual needs.
SIT UPS LOTS AND LOTS OF SIT UPS AND CRUNCHES
crunches
The different types of sit-up exercises that can be included in a workout routine are traditional sit-ups, crunches, reverse crunches, bicycle crunches, and Russian twists.
It all depends on how many you are doing, also keep in mind that sit-ups aren't as effective as crunches, crunches will get you abs much quicker... also, you must be exercising fairly frequently (read everyday), not just doing sit-ups, if you want abs.
Curl ups are an important exercise for toning and strengthening the core. They are also called sit ups or crunches.
bike crunches, sit ups, running .. good luck :)