There's lots to mention re: abs.
First something that's fact: you want at least one day of rest between ab workouts. They NEED this time to recover.
OK so now to the debatable stuff...
There's one school of thought that says "you wouldn't endless reps on any other muscle group so why with abs?" and another that says abs are a bit different so require a lighter load for greater reps (up to about 25).
I'm not sure which is correct and anyone who comes in here stating that one is, doesn't know what they're talking about. I would suggest that if you've got a swiss ball at home and are prepared to do several workouts a week, you can just do both.
So... I'd do something like weighted hanging leg lifts and weighted decline crunches for 5 or 6 sets total, aiming for 6-10 reps in each set.
Then at home on the swiss ball maybe look at 4 sets of crunches until you fail. Make sure you keep perfect form and your arms in the correct place - this is key.
Hopefully mixing the two types won't be counter-productive
If you're doing nice heavy sets for bench, deadlifts, squats, pull ups/chinups, etc, you'll be working them hard anyway. And it's not *that* hard to build ab muscles. The hard part is getting rid of the fat that's hiding them