I have both 21 Day Fix Extreme and Hammer & Chisel and here are my thoughts on both:
Hammer & Chisel Thoughts:
1) They're all total body workouts but some have cardio moves too (there's always a modifier or a modification is shown).
2) The Hammer workouts you can use heavy weights but the Chisel workouts are hard to go heavy because of the compound exercises and the speed (but you may not want to because they have a pretty good cardio effect).
3) ALL the workouts have lower body exercises (I had to start putting in some cardio every few days to give my legs a break).
4) Sagi and Autumn did the "extra" bonus cardio workout and the two add-on workouts together (no background exercisers). The cardio workout is a trainwreck, lol. Good thing it's only 17 minutes long! The add-ons are good total body workouts though, but again you have to get passed their dynamic together alone, lol.
21 Day Fix Extreme Thoughts:
1) Similar in set up to the original 21 Day Fix (plyo/upper/lower/cardio fix extreme/pilates with a band/yoga).
2) There is always a modifier like "Cat" in the original 21 Day Fix. She's great and easy to follow.
3) The warmup is only one time through (unlike the original 21 Day Fix). It's about 2.5 minutes long and may not be enough. You may want to do it twice.
4) The 10 minute ab section on this one is more advanced than the original. It isn't done twice through.
5) The add on workout "ABC Extreme" is one of my all time favorite of the 21 Day Fix workouts. You do the exercises in this order "abs, butt, cardio" for about 15 minutes, then you take a short break and repeat the exercises again with less instruction so it moves a lot faster.
I like both sets for different reasons. Hammer & Chisel is really a great set that gets ALOT done in a short amount of time. Most of the workouts are about 35-37 minutes long. You use weights in most of the workouts so they have a metabolic effect. It's also like getting two sets in one because you can use just the Hammer or just the Chisel workouts on their own.
21 Day Fix Extreme is a nice bump up from the original 21 Day Fix. It's not as tough as Hammer and Chisel but it's challenging for sure. I like to use these workouts on their own for some variety in other sets.
I think it comes down to what your likes are in a workout. You can't go wrong with either set, in my opinion.
Kathy