Are the abdominals better trained with low reps with heavy weight added, or with high reps and no external resistance?
Search the articles on this site, it has been a long debated topic by many well known strength coaches. Here is one article in particular:
http://www.t-nation.com/readTopic.do?id=459563
I personally combine the two worlds. I do ab workouts four times a week (many people may say this is too much, but i enjoy my results). Here is a sample:
Monday/Thursday - 4 x 25 (back to back, twice through) of low to no weight crunches, leg lifts, static exercises.
Tuesday/Friday - 4 x 12 (back to back, twice through) of heavy weighted decline cruches, dumbbell russian twists, cable crunches.
The way I see it is that neither side can come to a conclusion of which type of resistance is superior. At the same time, both methods are shown to build muscles. So until one type takes the crown, do both!