Drums handle stringy material better than discs, and they're almost always alot lighter to tow. They can produce chip quality quite comparable to discs, but you have to be very precise with the knife and anvil settings. Also, a drum will chip small non-forked stuff faster than you can keep up with. Bear in mind a 12" drum won't necessarily handle 12" material... it's merely a measure of the drum's width. If you plan on selling stuff 5" or over for firewood, this is likely a decent choice. Drum chippers are usually cheaper to obtain than discs.
If you plan on chipping alot of forked material, a machine with feed wheels will make you alot happier. Disc chippers also don't "thrash" material around the infeed chute as much. They're more tolerant of knife and anvil settings, and the knives are usually easier to change. If you plan on chipping larger material (up to 12" usually) you'll want the bigger capacity that mosts discs have over drums. They're more expensive, but if you can afford one, go for it.