Thats a good question. A whle ago I put that to a friend of mine whos a personal trainer. He told me that on the one hand your abs are just like any other muscle and would respond better with higher weight and lower reps. But he also said that the abs are conditioned for muscular endurance as they are working a lot of the time to support our spine and for posture etc.
I try to cover all my bases and do crunches (as many as I can do) and use the ab machine and cables and use heavy weight with 6-10 reps.
As Dr Evil said try both methods and go with whatever gives you the best results.