We have HIIT in my gym too, but it conflicts with my work schedule so I cannot do it. You are right, P90x and HIIT will make your abs more defined and burn more calories, but again, here is one major difference I discovered, by only doing P90x and regular crunches, you will get a six pack but they will appear as if someone drew it with a marker, the 'mass' will be missing. 'Weighted' reverse crunch, regular crunches, planks and oblique workouts will get you an Ulysses Jr. type abs.

I almost forgot to mention the 'form' during workout, a right form will get you instant results, I mean I can totally lift two 75lbs dumbbells and do a shoulder press but then I will twist and arch my body which means I will be working our maybe my lats or forearms while trying to lift that weight. Every workout should be done in such a way that your muscle is isolated, several years ago while doing squats I was actually working my lower back than my quads, I was working out so wrong. :(