I would go the compressor route, as you can far more tools to use with it. Any impact guns should state the pressue required to function, so get a compressor accordingly. I have an 8gal one which seems to do most things . . although some tools run out of puff quicker than others so you have to wait for it to charge again.
Also if your not in a rush, just wait for compressor or tools to come on offer, sometimes you can pick up a bargain.