I say it would depend on your current fitness level, your history, and how much time you're willing to dedicate. P90X is a huge time commitment (1 hour or more a day for 6 days a week) and most people don't want to, or can't, spend that much time working out. It's also mentally challenging to stay focused when you're working out that often, meaning you stand a better chance of losing interest and quitting. If you're a beginner or haven't worked out in a long time, I would suggest starting out with simplefit, just to get into the swing of things. Simplefit workouts are short so there's not excuse not to do them. They'll also get you used to working out against the clock, if that's something you've never done before. By the time you're at L5 or L6 I think you'd be adequately prepared to jump into P90X. Many people on this board have used simplefit as a springboard into other more advanced programs.