If you want to buy a new welder, Miller, Hobart and Lincoln are the best choices. Pretty much in that order too. Miller's are more expensive, but you really do get what you pay for in this case. Will they all lay down equivalent welds? Pretty much. But, if past machines are any indication, Miller's will outlast all of them. Not to mention that Miller still supports almost all of their old machines.
On that point, buying a used welder isn't a bad idea at all. Old Miller's tend to have better duty cycles than their modern equivalent's, and you can get them for a lot less than new if you know what you're looking for. A lot of people out there buy some pretty nice welders, use them a couple times, and then sit on them until they sell them.
I have a 1972 Miller 320 AB/P TIG welder and a mid-seventies Millermatic 35 MIG. The 320 AB/P has a 60% duty cycle at 200 amps! And it will stick weld and serve as the power unit for the powcon plasma cutter that came with it. I bought it for under $400, and it came with everything, including a water cooling unit, brand new torch, a bunch of gas welding supplies and the previously mentioned plasma cutter. Heck the extra long stick welding leads (20ft+ and super heavy duty) that came with it are probably worth almost as much as I paid for the whole mess in copper alone. You have to be a little more hands on with setting it up, ie, throwing lever switches and messing with a rheostat dial instead of pushing a button, but there's nothing digital on it and I suspect it will probably last forever.