If done correctly, there is not a better or worse. Both work abs. One reason that most fitness professionals have moved away from sit ups is that you only work your abs for the first part of the exercise then you start incorporating other muscles. Also, there is more of a chance of back strain if you do sit ups wrong.
If you are looking at getting muscle definition, you must first have the physique to do it. If you are bigger in your waist area, then you will have a tougher time getting that six pack than if you are a thin person. There is no such thing as spot reduction. By doing situps as part of your diet and fitness, when you do lose the weight (if this is the case, it is not part of your question), you will have the 6 pack abs to show for it.
If you are incorporating situps in your exercise routine general toning or strength I would continue. Also vary it up by adding weight (i.e. hold a med ball while doing crunches or hold a med ball between your knees). Variation is key to a good program.
There is a place for both in a routine but there is more risk in doing sit ups.