Serbu stopped making Maverick Super Shorties several years ago, to my knowledge. It's all 500s and 870s now, and the 870 only costs $50 extra. I see Remington 870 Super Shorties in videos on Youtube pretty regularly, so they aren't uncommon. They probably cost more because the 500 only needs a plug welded into the end of the mag tube, while the Remington has to be re-threaded.
There are also Remington-built AOW 870s available in breaching configuration (10" barrel, Pachmayr grip, top rail), and "witness protection" configuration (14" barrel, bird's head grip). Autoweapons.com is the only dealer I know of that regularly stocks them, and their prices are always very high. But they're definitely available.
Personally, I prefer the ergonomics of the 500. Slide release, elevator, etc. I'm completely happy with my M500 Super Shorty.
I'd say you can't go wrong with either make of Serbu for your first AOW. Serbu has great customer support, it's the most compact shotgun out there, and both brands have about a million accessories available.