From memory I can tell you that I prefer the Pro 500s all the way.
Even COMFORT (which I hate on the Pro 500s) is better... ;-)
Sound is different all the way.
The Momentums are a bit more "laid-back" due to their warm nature and it does have kind of neutral mids but imaging is not the best, the sound is too congested up-top. I don't remember that being the case on the Pro 500s even if it had recessed mids. Also, of course, the Pro 500s have HUGE bass. It sometimes bleeds into the lower mids but it doesn't create a muddy mess.
All in all, sound-wise, I liked the Pro 500 a lot.
The Momentums were not my thing, in fact, I even like the Momentum on-ears more!
If you can try them, I implore you to do so.