The difference between a set with aluminuim windings compared to copper is very great, and the copper wound sets are far easier to use.
I'm not convinced. About a year ago, I searched the Internet for information on the pros and cons of aluminium vs copper windings in various transformers, including some quite critical industrial applications. I concluded that there's little difference. The heat transfer quality of copper is superior but aluminium is also a good conductor. As for electrical differences, it would be interesting to see if anyone could tell the difference for welding, all other factors equal, such as the wire feed mechanism. I'd wager that in a blind test, you wouldn't be able to tell the difference.