I have a Sega Genesis Model II and a Sega Genesis Model III. The only difference between the two is the size and the fact that the Gen III can't use any peripherals like the Sega CD, Sega 32X, or the Game Genie. I use my Genesis II more often because it has more of a nostalgic feel to it and if I feel like it, I can use my Game Genie. If I'm showing off the Genesis or taking it with me somewhere, I like the Genesis III better.
Sega Genesis III Pros and Cons:
-600+ Games for the Console (something for everyone)
-Sleek slim design (looks nice next to a PS3 or XBOX 360)
-Uses all previous controllers
-No longer made (you can only find it in a resale shop or on ebay)
-Rarity and Price point ($30-$70 used; I got mine off of ebay for $35)
-No new games (but with 600+ games, if you can't find something you like, then don't buy it)
-Not compatible with Sega CD, Sega 32X, or Game Genie (Game Genie is good for cheating, but what were Sega CD and Sega 32X any good for?)
In my opinion, the pros far outweigh the cons. But, If you want a smaller, Sega-branded model of the Genesis, I suggest the Genesis III. If you don't particularly care, I would suggest getting a Genesis II. This model fixed some of the common problems with the original model (integrated circuits, better a/v system, universal rf, a/v, and a/c adapter cables for the entire Sega Genesis line) and ended up being a great product through out the 90's.
If you would like a brand new Genesis console or are looking to completely upgrade your current Sega Genesis, I would suggest you pick up the Gen X Dual Station console.
Not only does it play Sega Genesis titles, but if you never had the chance to experience the NES or vice versa and you want the best of both retro worlds, I would suggest picking it up. In the end, it is just a redesigned Genesis III with an added NES function, you can use Game Genie on this model, and you can also play import Mega Drive and Famicom games (JP and UK) with the flick of a switch.
The company that made this also took the liberty of putting standard RCA cable jacks (red, white, and yellow) to solve any previous problems in hooking your Genesis console up to a newer TV. On top of that, the console includes two six button controllers so you can play two player arcade style games like Street Fighter II or the Mortal Kombat series right out of the box.
I've seen a demonstration of this model on YouTube and I might have to pick one up this summer to add some longevity to my retro gaming hobby and also catch up on all the supposed NES classics I might've missed...^_^