I asked the same question
http://forums.pelicanparts.com/paint-bodywork-discussion-forum/485108-millermatic-140-auotset-worth-extra-money.html
The work I did on my 911 was handled very easily with the 140, the voltage very rarely was turned up past #2 or 3. I didn't know it can be used on carbon or stainless. I am still learning myself. I would find a dealer that sells them and go in to talk with them. The salesmen are usually welders themselves and can answer a lot of questions. It is also handy to have a shop or person to go to when you have questions, they bailed me out several times helping me with technique etc.
If you plan to weld big and heavy I would go with the 180. Hooking up a 220 source is not a big deal.