Pro M vs. Yamaha? Well, I would probably choose the Yamaha, if the bearing edges are in good condition.
I havea Yamaha Tour Custom (1987), birch. I'v eused this kit on a ton of recordings and it has never left me high and dry. Now I know you're considering the Stage Custom, and they are two different kits, but I like Yamaha. THat's pretty much it.
The Pro M by Mapex is cool, in fact, Mapex offers pretty great deals. I worked at a local retail store (WLA Music) and got a good education about drums from the department manager, Frank Eck (really, the dude knows about drums). The Mapex kits always sounded surprisingly good (especially the Saturn kits) and we cost effective. The Pro M kits had great finished (wraps) and a nice vibe.
The best way of knowing if you like a kit is to simply play it. If you have the opportunity to do so, hop on it! I've heard supposedly bad kits sound great, and great kits sound bad. Consider you application too. If you are recording, think about what will sound great on the discs you make (why I love my Yamaha's...birch records with fewer mid frequencies to manage from the engineer's standpoint).
Go check both kits out, look at the bearing edges, and play them. Your heart will know which one you should take!
Musicality is 97% more important than technique...