I'm an avid 4WD person, but I have not been "in the market" for a while now. I currently have an ARB on-board air compressor (the larger one- I understand there is a smaller one on the market now)AND a Quick Air 1 in my rig. Obviously, the primary use for the ARB is to power my air lockers, but I also use it to air up after a trail run. Both of these units combined cannot compare to the ones that could power an air tool such as an impact wrench. However, using both at the same time makes for a faster "air up." I installed an extra 12V socket at the rear of my vehicle to accomodate the Quick Air.

Knowing what I know now, I would first consider what your primary purpose would be. The units that have the ability to power an air tool, to me, would be the best choice because of its versatility- you basically could do anything air related such as popping a tire bead back onto a rim and anything less. The smaller units just don't have he grunt to do that, but you get what you pay for.

If you're looking for an electrical unit, look at the duty cycle. Many compressors have a "time limit" before you have to allow it to cool. Again, that's going to depend on what your primary purpose of the compressor is in the first place.

There are engine powered ones, but that might be over the edge-I understand they're awesome though.