I have both sets.
Here's my take on P90 Masters:
The workouts are similar to P90X, but with some new moves and the old moves used in new sequences. Ilove that the cardio is very drill-oriented and athletic, no choreography here!
Intensity is sometimes close to P90X, but the workouts are shorter (like the cardio ones, which sometimes seem too short). It is easier, though, to combine workouts. I've done this by doing Sculpt 5-6, then after an hour break to refuel and recover, Cardio 5-6. I'd say the intensity is about the same as Power Half Hour workouts.
It is not set up as a rotation, the way P90X is, and there are plyo moves in all the cardio workouts, so it could be too much to use them as a rotation.
I really like some of the P90 Masters workouts (Sculpt 5-6, a total body resistance workout emphasizing upper body; cardio 5-6; Interval cardio; plyo legs). BUT, for the first time, I've encountered a Tony Horton workout that I don't like and am getting rid of: Core cardio. It's a mix of standing cardio and floor core work, but the core work isn't as innovative as some of TOny's stuff, and I absolutely hate going from standing cardio to lying on the floor for abs (which is something many instructors say NOT to do!)
I still haven't tried UML (upper-middle-lower) but I like it based on previews.
Overall, with the exception of Core Cardio, I enjoy the P90 Masters workouts.