21 day is for beginners. More rest between sets, easier work out. Lots of modifications
Extreme is much harder. Uses the some of the same moves as 21 day Fix but no modifications. also much less rest, more intense.

My suggestion, to start with 21 day. Do a few rounds, then when you feel better (like doing no modifications, and can go the whole workout without stopping) to change

I did 2 rounds of 21 day fix and started now Extreme just finishing round 1 yesterday. I will have to do Extreme a few times, I cant get thru the workouts to date!

Good luck! I have both but I have never progressed enough to actually try the extreme.