I highly recommend a 2 stage air compressor. It's more efficient in air pumping/ energy and typically a lot quieter. your ears and neighbors will thank you. You can read more info here (or do your own searches, too):
http://www.ehow.com/about_5103936_tw...ompressor.html
http://www.ehow.com/how-does_4913962...sor-works.html
http://www.popularmechanics.com/home...icient/1275131
I'd also recommend (as mentioned already) getting the best one your budget will allow. Larger tanks hold more air/ volume at the same psi so you'll get more use out of your tools before refilling. As you shop look if it comes down to a couple of choices I'd also consider the one that runs at a lower rpm to make the same air psi/ volume (typically means a more efficient compressor pump) as well as less noise, power usage, etc. I'd also consider the brand. name brand/ major retailer will likely have a better warranty, easier to get parts or service replacement, etc..
Understood about your space issues, but if you get a vertical tank you can put it in the corner and plumb some pvc or copper piping throughout your garage to make multiple air fittings. Then you don't need to lug your compressor out every time you wish to use it. Just turn it on, plug in and go. They also have a smaller foot print so they should consume less space.
I started out with one of those single stage 33 gallon 6 hp sears craftsman units. It was loud, vibrated when running (would literally move around and not stay still) and could only temporarily run an air grinder. It really wasn't powerful enough to handle my current firebird project. I recently stepped up to a home depot 80 gallon 220 volt (single phase) 2 stage compressor. It's powerful enough to run my sand blaster cabinet and air tools all day long. I hope it's the last air compressor I will have to buy.
The single phase (not stage) has to do with how it is wired up/ 220 volt circuitry.
I hope this helps.