I would (and did) go with the Millermatic 175. I test drove a few welders and found that the 220v just welded better. It struck a more consistent arc, and when welding thicker stuff it really had the heat to do a good job. With the infinite voltage control, I have welded as thin as 24ga with it and as thick as 1/4''. It works like a champ.
The 110v welder could do 1/4 inch if you properly prepare your joint (make a 45 degree bevel at the edge to thin it to about 1/8'' and weld in two passes).
As for plugs, The MM 175 uses a plug like the second picture above. I'm not sure if a 20A breaker is going to cut it though. I'm running mine on a 40A breaker.
Good luck!