the guys that run a lot of aluminum wire - like manufacturers of stock trailers - have a gadget called a "pulse box" that's pure magic. every mig machine i've run aluminum with - usually with a spool gun - creates tons of spatter and ash, no matter what gas you run. it's an ugly weld and a bitch to clean up. the pulse box basically turns the globular deposition into a spray deposition, and the welds are perfect! oh....the catch is that equipment is well north of a $1000 just for the box.
soooo....unless you're going into mass production, stick to the tig machine. most welding supply houses have a "magic gas" blend of argon, helium and some other stuff that will weld aluminum really nicely for just a little higher cost. oh, and don't use anything but stainless steel brushes to clean the metel before you weld.
have fun!
wp.