I recently did some testing between the new 180 auto set, 212, 251. The 180 is just not enough machine, so that leaves the 21 or 251. The 212 has a much more stable arc than the 251 due to the transformer that they use in it. The 251 and the 180 use the same style transformer. The spool gun of choice would be the Miller 30A, due to it;s duty cycle. The 30A is a 200 amp 100% duty cycle, with the gas coming in thru the handle, much higher quality (made in the usa). The spoolmate 3035 and 3545 both have a 60% duty cycle and feed gas at two locations one at the front of the gun and one thru the plastic handle, that WILL overheat.
After doing a fair amount of research and testing I Recently purchased a new 212, to replace my old Napa body shop welder, that had spot, stitch, and normal welding features. I also have a Miller HPF250 tig (water cooled) that has been working flawlessly since about 84 when it was purchased new.