good advice already give re: 6" chippers being pretty much a waste of time, and chuck n ducks being too dangerous. You might get better advice if you post more info about what sort of jobs you're doing, and how big your truck/chip bin is.
In general, bigger is better. The XP90 is not a bad machine, and I'd take bandit over a vermeer because the parts are available everywhere, for cheap. You can pretty much rebuild one from any auto shop, same story with the engine parts, clutches, blades etc. The computer that runs the auto feed is probably one of the few proprietary parts, and its $$$. Different story with vermeer, most of the parts seem to be proprietary, so you've only got one place to buy and they're gonna stick it to ya.
If you look around you may find a half decent XP150/200/250 within your price range and those are all real workhorse machines. I've got the 250 with the 120hp perkins turbo and it goes great. Weighs about 6,000lbs, single axle and tows pretty good. The infeed is 12"x19", so it swallows some good size limbs without fuss. Throws the chip out real fast and hard, packs more into a chip bin than most drums can. Nice consistent chip size too, you just gotta keep your blades sharp and anvil adjusted.
9" is ok, but you'll be real glad you got a 12 if you can.
Shaun