I have a Millermatic I sort of "borrowed" from my girlfriend's dad about 2 years ago. It has the capability to run gas, but I've been using flux-core for all my stuff.
It's a 115 volt, and welds anything up to 1/4'' or even 5/16'' just fine, all the way down to about 16 gauge without too many problems burning through. 18 gauge is tough without burning up the metal.
I would just get the 115 v if I were you, unless you really want to weld some steel beams or something. I made my bag brackets out of 1'' square tubing and 1/8'' plate, and some 3/4'' bolts I cut up. I could post a pic if you want.
The lil' 115v mig welders are pretty dang useful, for any project you may have.
We welded a big ass steel cage covered with expanded steel and a heavy steel gate inside my buddy's band's van so they can lock their equipment inside. The welder was running for 3 days straight working on the cage and never missed a beat. The cage is awesome, and totally iron-clad sturdy.
I'd also recommend getting a 4'' angle grinder, with the cut-off wheels/disks, and some grinding disks, and some of the sandpaper covered flapper disks. I have a Ryobi angle grinder that was less than 50 bucks and I've beat the hell out of it for 2 years and it works great. My favorite tool!
With a MIG welder and an angle grinder, and a good vise you can build almost anything.
I think the Lincoln welders are pretty good, and like you said are readily available at the big box stores.
I'd get the nicest 115 volt that can handle gas, or maybe the 240 if you have power available and just want to.
Really, after spending time with mine I wouldn't get a nicer one, I'd spend the big bucks and get a TIG. Santa???