Assuming you don't want to do anything weird, that autoset is pretty useful. I generally don't trust stuff like that, but it actually seems to work well. And, if you do want to run multiple passes or run super hot on sheet you can always turn it off.
Just to throw this out there, though, I've used little Lincolns for offsite work before, and you can really stretch them if you need to. I was on an installation on Monday where we were using one to join 5/8" flat bar to 1/2" flat bar with .030 and straight CO2. Crank the heat, turn the wire feed super low, and they'll burn in more than you think. The same may be true of the Millers, but I haven't used them that way myself.
Both will last you years, I'm sure. More important than the machine you buy, though, is time under the hood. If you want to get to the point where you can trust your welds to hold set aside some time and some wire and gas to practice for a few hours a day a few times a week. Get some scrap (often steel yards will have drop they sell by the pound) and weld and weld and weld. With MIG especially, pretty isn't always strong. Focus on getting solid penetration on both pieces of metal and then you can worry about laying down something that looks like they do on those auto "reality" shows.
