The 92 version is the one already setup to take 92FS/M9 magazines which are readily available, plus when the military ever does start to change over to a new sidearm all there leftovers should go up for sale at sometime. The other 9mm versions are already setup in the mag well to use either Px4 magazines or Cougar magazines. The coversion to one or the other is about as easy as changing a mag well adapter from what I know, but if you have any of the pistols than buying the model that matched is the smart buy of course. I bought a 92 version first having always loved the M9 from the Army and 92FS in the years before, but hadn't bought my own yet. I liked it right away as it's very accurate, has super light recoil, is quick on target and very reliable. With an Aimpoint or Eotech it will be very fast on target as it's so light at 5 3/4 lbs. I ended up buying a commercial M9 to go with it, realized that I have always loved the M9's because they fit me so well and I shoot them so well, so I sold my P226R that didn't as much and bought a 92FS besides. The 92 mags are the easiest to get, well priced and I'm sure to be better in price in time, and there are 30 rounds mags available.
heThe 30's are Promag which I don't have and have read 50/50 reviews on, so use them with a grain of salt till they prove themselves, but that's any mag really.
http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=825087
http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=938507