I would definitely go with a Mid-Length. Smoother action and gives you more hand guard real estate. If you're wanting the most accuracy, stay away from chrome-lined, they are slightly less accurate. If you're expecting to eventually be sending high round counts through it, go with a Chrome Hammer Forged.
You can get them either as a barrel assembly, (barrel, barrel nut, gas block, gas tube), or just a stripped barrel, whichever suits your needs.
As far as brand, I'm probably not the best person to ask as I'm not really an accuracy type guy, but BCM makes quality stuff and they sell all sorts of configurations in both stripped barrels and assembly at pretty decent prices.
"Breathe when you can, shoot when you should."
"Qui desiderat pacem, bellum praeparat; nemo provocare ne offendere audet quem intelliget superiorem esse pugnaturem"