(The following is from personal experience, and may not apply to others)

Insanity:

Benefits: Huge calorie burns, and when you're done, your cardiovascular conditioning will be pretty darn good.

Downfalls: The workouts are difficult. Very difficult, and to some, that can be discouraging. The intensity of the workouts is pretty punishing, and things do tend to ache, especially feet, knees, etc. Make sure you have the appropriate sneakers and insoles before starting. Insanity is definitely all about the cardio and fat burning, but you'll also be burning the muscle you built during P90X. In order to maintain your muscle mass, you're gonna need to dial in your diet just right.

P90X3 (Disclaimer, only did P90X3 a couple of weeks, so this opinion is based solely on those two weeks)

Benefits: Shorter workouts. About 30 minutes for each. Kind of a blend between P90X and P90X2, with a lot more synergistic exercises (working multiple body parts at once). If you get your diet dialed in, I'm pretty sure you can maintain and/or improve on the results you achieved with P90X.

Downfalls: Shorter workouts equals less calories burned, so you'll need to adjust your diet accordingly, which is why I stopped doing X3. I like to eat, and the thought of restricting my diet another couple of hundred calories a day was a real drag. Also, X3 Ab Ripper doesn't come with the base kit. A lot of the exercises are core centric, so it shouldn't be a problem, but P90X and Ab Ripper go together like peanut butter and jelly.

If I had to choose between the two, I'd go with the program that targets my overall fitness (strength, cardiovascular endurance, flexibility, etc.), which would be X3.

If you want to lose more weight, I'd go with Insanity.

If you want the best of both worlds, go for a P90X/Insanity hybrid.