pneumatic is the way to go if you're willing to buy a compressor, if not then electric will work but dont buy cordless. A stubborn lug will drain that battery in a heartbeat.
I'm sorry, but none of this makes any sense. If you want a proper impact, you need a pneumatic, compressor setup, etc.
But, if you're going elecrtric, there's no reason (in 2013) not to get the cordless, because you're going to have the flexibility to take it to a breakdown and use it there.
Corded, plug-in electric impact is clown shoes, IMHO.