We use a Lincoln MIG with a spool gun. I don't know the wire size off the top of my head though. We bought the spool gun in 2005, after trying to weld aluminum without it. The smallest wire we could get was just too large to work with, and we kept having problems with the feed. 100% Argon in the tank.
The spool gun is much easier to work with, and much easier to learn than a TIG. The welds aren't as clean, but with enough practice, they work just fine. TIGs are great, but take a lot of skill and practice to do correctly. All of 1388's robots have been MIG welded on site at our school.
Here's an example of our MIG'd 2008 frame.