CO2 for me... though I have an ARB compressor that I use for primarily for the Air Lockers. Both are rigged to function as the other, redundant system. The York is sweet, but I'm honestly yet to run across a need for that much air. Air tools? Well, I've carried them in the past for use with my CO2, never needed them. There isn't a single bolt on my Cruiser that won't come off by hand? Mabey my crank pully but if I'm pulling it off on the trail there are bigger issues.
Some of the cheap 12V compressors are really impressive, Carl mentioned the Puma, I too have heard some great reviews. The new ARB compressor has double the CFMs of their older unit, and thereis rumor of a larger compressor from them in the future.
The portability of the CO2 has been a proving factor for me... I've hauled mine back up the trail to a stranded rig, or in the house to blow out the DVD player, or filled tubes with it when floating a river... All things a York isn't so handy at. I've never run out of CO2, afterall its really easy IMO to check your level (before you head out on the trail). Weigh it... the tare/full weight is marked on all my tanks, so I know exactly how much I have. I have 3 different tanks, so one is usuall full enough to use