I assume when you say "tank" you mean a CO2 tank? I kept debating the same thing you are and finally went ahead and got the ARB compressor. It may not be as fast as a CO2 tank, but it never needs refilling and it's half the price of a CO2 tank (unless you build it yourself). Maybe down the road I'll get a tank, too.
Figure it this way - you get a CO2 tank (10 lbs). You fill up your tires and invariably someone else needs airing up, too. (I think T and Will can attest to this). Not sure how many times either has re-filled their tanks, but it's $7-10 US each time. Refill for an ARB - press of a button.