They are quite different IMO. I'll try to keep this one short and sweet:
Ballet Body:
- less reps / more moves
- requires (a lot of) strength (Planks, etc.)
- no specific cardio exercises (but HR rises during some segments)
Ballet Beautiful:
- 4x8 reps for most exercises
- lots of floorwork (but also standing work)
- cardio exercises
I really like Ballet Beautiful - I find it kinda relaxing but still challenging but I can do all the reps. Ballet Body on the other hand is really challenging for me because of the required strength - when I first started I wasn't able to do her Plankwork at all - I had to build up to it vs Ballet Beautiful where I could do everything from the get go.
Ballet Beautiful is really great for the inner thighs, butt and arms (most of that is done without weights and standing so no planks at all) - the cardio exercises require some balance and but they are not that challenging concerning specific cardio but some are higher impact.
Ballet Body is great as well - it really defines my muscles but some people have proplems with thigh bulking (all those plies).
The exercises from both systems are completely different - really not comparable - but the complement each other really well.
Ballet Beautiful is more gentle while Ballet Body might be even grueling at some times.
It really just depends what you want - they both give results just in different ways and I uess it depends what else you're doing.
Hope that helps - and sorry I guess I'm unable to do anything short and sweet
ETA: Ballet Beautiful segments are mostly 10-15 minutes. Ballet Body segments are mostly 20+ minutes. DVDs of both are around an hour (Ballet Body more around 45-50'). Ballet Body Periodization Systems are between 60-90' (BBPS II 60' which is the only one I did). Mary Helen says to do her workouts for at least 3h a week - Leahs system require you to follow her plan from 3x/week to 5x/week and they take around four (BBPS II) to six (BBPS I) months.
ETA 2: I find that while Ballet Beautiful somehow slims me down it doesn't really give me muscle definition which Ballet Body does - it also doesn't give me strength (at least not really noticeable, of course the segments get easier etc. but it's not like I notice strength gains in another context), Ballet Body really does work as a Bodyweight/strength workout where you notice your strength gains.
ETA 3: Also,... Ballet Beautiful has a set of exercises like Bridge, Swan Arms, Tendu, Arabesque,... - Ballet Body has a lot of variety. That's not to say Ballet Beautiful is repetitive but I find Ballet Body to be more... creative.
Ballet Body has more standing work - Ballet Beautiful is mostly done on the floor for toning and standing for cardio (though there are exeptions).