http://www.gasguns.info/forum/viewtopic.php?f=8&t=13405
The M9 and 92FS is the exact same pistol. The only difference is the markings on the slide and seeing how most airsoft Berettas don't come with real markings anyway it's no difference.
M9 is simply the military's own designation for it. 92FS is Berettas designation. M9s are 92FSs. The military just uses their own naming system for record keeping and inventory reasons.
Just like the military calls it a M4A1, but Colt calls it an Colt 921. Same rifle.
As for who makes the best? Depends what you describe as best.
-WA: I wouldn't consider the WA much. They're nicely made, but pretty expensive for an all plastic pistol of iffy durability with stronger gasses and limited upgrade potential. Working decocker.
-KWC: No experience with them. I hear KWCs tend to be pretty robust, but kinda outdated designs and with practically no upgrades available. There's better designs out there for the same money.
-KWA/KSC: They make good guns, especially the new NS2/System7 models. Not much aftermarket for their m9s but they are good enough that there isn't much you need to do to them. Good guns as long as you don't plan to change it much. Working decocker. RA-Tech seems to be working on a steel slide for these.
-Old Marui M9: It's a Marui, 'nuff said. Good accuracy and range, good efficiency. Not the most powerful and made of plastic, but enough aftermarket parts available to do anything you want with it. Guarder has steel parts available and it's the only design you can get a steel slide for from SAT (for $500 though ouch) if you want a bit of extra authenticity. No working decocker though kinda lets it down a bit.
-New Marui M9A1: Released not too long ago. Adds a working decocker and other little changes to update things a bit like aseparate extractor and rear sight. But a new design that's mostly not compatible with aftermarket parts for the older M9. Shooter Design makes an aluminum body kit but it's pricy. I'm sure the aftermaket will grow soon. Only available as a railed M9A1 variant so far which is just ugghhh :sick:
-KJW: Pretty much a straight up clone of the old Marui M9 but with potmetal body instead of plastic. Probably the most popular M9s out there. Compatible with all Marui parts, built like tanks. Cheap paint finish looks kinda lame though. Good if you want a rugged, if ugly skirmish gun and don't feel like paying more for the Marui. Inexpensive base to start upgrading from.
-Old WE: Also a clone of the old Marui design. Hear they wear out and break pretty easy. I'd avoid them and just go with a KJW if you want a cheaper sidearm.
-New WE: Clone of the new Marui design. Full metal. Also with working decocker, but WE made it select-fire for some stupid reason. The fit and finish on these is pretty good. I hear newer WE's are a huge improvement over their older designs.