I have the MM175 with the Miller cart and mine sits loose too. I used one of those rubber mats for tool box shelves so it will stay put. A friend of mine has the harbor freight cart and I think it's much nicer. It places the welder closer to arms reach, which is a great feature right off the bat. Also, it comes in blue that's very close to the welder's color. One thing however, with the HF cart, IIRC you need to "notch" the table so the welder's door can open. Not a big deal.....
As for the original topic: Miller:Hobart:Lincoln...? As others have already stated, on the 110v and even the 220v units all three make good models. You really can't go wrong with either.