In short, if I stayed with the MD3.2s, I would've been happy. Possibly one of the best values out there (especially when purchased used). But I've recently found a super deal on the MDP and immediately sold my MD3.2 shortly after.
Soundstage is the first thing you'll notice, it's about 35% wider. The sense of space on the MD3.2 was no slouch either, the Pros are just phenomenal for a closed headphone. Bass has gotten noticeably tighter, and whops a stronger punch. It may not be enough for some, but for me, it's perfect for EDM. The MD3.2 was fairly decent at EDM, these are way better.
Clarity and microdetails has gone way up too. Sometimes I'm not sure how I feel about this. The MD3.2 had a nice lush smokey flavor to it... it's more laidback and I enjoyed that aspect of it. The Pros lose some of that, and even though it is technically warm, it's fairly brighter than the MD3.2. The treble is definitely more present, and could be sibilant at times, but Dan provides doggie treats (pads and tuning dots) to help remedy that.
Am I happy with my purchase? Yeah, I found a killer deal on them and they're the logical upgrade to the 3.2, without having to go to the drier alpha dog sound. But used MD3.2s show up on 'for sale' for about $150-180, and I think that's one of the best deals out there. You can't go wrong with either IMO.