i use a mix of both. steel mags are deceptive if you've got them mixed in your gear with aluminum, because an empty steel mag feels like an aluminum mag with a few rounds in it.
one thing i like about the steels though is that they don't swell over long term use. i hate having to squeeze the sides in on an aluminum mag that's too swolled to fit in my mag well.
i haven't had a problem with rust on my steels, but i also take pretty good care of them. i will be purchasing a shitload of aluminum mags though, when the ban sunsets. i don't mind having a few steels, but i'm probably going to make all my mags in my gear aluminum, for uniformity and weight issues. 12 steel mags would weight a bit more than 12 aluminum mags.
price-wise and durability-wise, steels are better if you keep them oiled. but if i'm putting something in my gear, it's something i don't want to have to oil. because who has time to sit down and oil all their mags in the field? and who wants to carry all the oil to do that?