AFAIK they all have aluminum windings and aluminum wiring. It really does not make any difference with this class of welder anyway.
AFAIK they all have plastic feeders with plastic gears. Not ideal at all, but no one offers a metal feeder in this class of machine.
And according to the table Outlawmws posted they all have a 20% duty cycle. Two minutes on, eight minutes off. For a hobbyist it's likely fine, but OMG I would cry.
I'm going to go out on a limb and say that all low end consumer grade welders will have limited factory support in the distant future, Lincoln, Miller, doesn't matter. But then the mechanical parts are probably all very similar. And the widespread proliferation of these cheap machines means spares will likely be available second hand. The quality of those spares... who knows.
Otherwise, play with all three. See which one welds the best for you. Compare warranties, and then go with your gut. If you think you will probably upgrade in a few years, go cheap.
If you don't need a machine immediately, peruse CL, the auction scene, and your local classifieds for a 220 machine. You'll get metal feeders, better regulation, better arc characteristics, the ability to use larger (cheaper) spools of wire, a more robust duty cycle, the power to weld almost anything a hobbyist or farmer would need, and long term parts availability.
The only thing I would look out for with a used 220 machine is the low end, if you intend to do a lot of sheet metal work. I know some of the older 250 machines did not go any lower than 30-40 or so, not ideal for autobody type stuff.