All the beretta 92's are essentially the same. The function is all the same, as are reliability, parts costs, ect.
Here are the notable differences:
Vertec: Straight up and down grip provides a slight smaller trigger reach. This may be helpful for people with small hands, though I personally just think it's ugly and uncomfortable. There are some indications that the model has flopped and that resale value just may not be there. If you PERSONALLY like the Vertec style, go for it, but don't be suprised if you dont get much out of it when you decide to let it go.
Brigadier: slightly shorter barrel that ends flush with the end of the slide. The slide is also heavier. The idea is to make a slightly more compact pistol and the heavier slide absorbs recoil. I have personally never dealt with a Brig, so I have no opinion on it. If you're interested, check it out.
EliteII: Higher end version of the Bridadier pistol featuring all the features of the Brig. It also features a skeletonized hammer and a D trigger spring (see 92D) and a 92G safety (decock only). The D spring contribues to a lighter trigger pull for a double action shot, and the skeleton hammer provides a SLIGHTLY faster followup shot, but most shooters cant even tell the difference. I have put the D spring and skeletonized hammer on my 92FS (the parts are interchangable)
Inox: Exact same gun as its black counterpart, but in a silver, corrosion resistant finish. If you like a silver pistol go for it, other than that, there's no real difference.
M9: Military version of the 92FS pistol. Functionally, mechanically, ect it is exactally the same. The only difference between the two models (92fs and m9) besides the numbers stamped on the side is that the 92FS has a 3 dot sight system, while the M9 has a "dot the i" setup, though the physical sight posts are the same, it's just a matter of paint. I personally own a 92FS and couldnt be happier with it.
92D: same function as the othr 92 pistols, with the exception that it is double action only. It is distinguished by its spurless hammer that lies flush with the back of the slide. Features a lighter hammer spring that gives a softer trigger pull for double action. Many 92 owners (including me) put the D model spring in their pistols to get the lighter pull for double action.
92G: Same as the 92FS with the exception that it does not have a manual safety. The switch on the slide is a decock lever only, and will spring back to "fire" mode after manipulated.
I would suggest going to a gun show and/or range and handling as many different models of the 92 that you can get your hands on and picking the one you like best. Good luck, and I'm sure you'll love whatever Beretta you decide on!