This is a very common question! To answer it succinctly, P90X classic is the better program, Lean should only be done if you think you won't be able to complete classic effectively.  Classic is how the program was originally designed - it focuses on specific opposing muscle groups and blasts them, then builds in enough rest time for you to fully recover.  It also has Ab Ripper 3x a week.  There are some great benefits and smart planning if you follow this schedule, such as doing Kenpo the day after Legs and Back, since Kenpo has an extended leg stretch at the beginning, and putting yoga far away from chest workouts since you will be doing a lot of planks.  The main difference between classic and lean is that lean subs in Cardio X for Plyo X and while you do core synergistics more often, you only do Ab Ripper once a week and don't hit every muscle group every week.  Cardio X is the easiest workout of the batch and the calorie burn is less than Plyo X, and by not doing the additional Ab Rippers, you really truly burn fewer calories per week with lean.  There is nothing about lean that will make you "leaner" than doing classic.  Many women especially worry if they do classic they will bulk up, when in reality classic isn't a bodybuilding routine, it is circuit weight training, and if you choose to do 12-15 reps with lighter resistance you will not bulk up.  The nature of P90X is moving fast between each weighted exercise and many moves are compound moves which don't give you strict isolation as a bodybuilder would, so even if you go for 8-10 you can't expect to bulk up, women or men.

So who is Lean for?  Ironically, it's not for cardio lovers, you actually do LESS cardio than in classic because Plyo is a longer workout that is excluded.  It is not for people that want to get "lean" instead of bulky.  It is really for people that aren't ready for full-blown classic yet.  If you cannot complete workouts like Plyo and Chest and Back on a regular basis effectively, Lean is an easier schedule to ease into.  Once you are ready for Classic, go for it!  I've heard other coaches call P90X Lean simply "P90X Lite" which I think is a great way to think about it!

So to summarize:

P90X Classic

-Works every major muscle group in a week with appropriate rest time

-Ab Ripper 3x a week, and Plyo X = more calories burned per week

-You won't bulk up, strength exercises are not meant to be bodybuilding exercises

-Might be too difficult for a beginner


P90X Lean

-Cardio X is a very light workout excluded from Classic which you will do weekly

-Ab Ripper only 1x a week, fewer calories burned per week

-More random schedule, but good for people not quite ready for P90X Classic. Many have still had great results!