If I was going on long trips with lots of corrugations, I'd enquire whether the alloy was more or less likely to crack.

The other issue with alloy is that once cracked, not all bush workshops will be equipped to weld it.

Despite the fact that I have a cheap heavy second hand steel rack on the roof of Silver, I'd prefer alloy for what I use it for.

The weight of the rack eats into your total allowable roof load, as well.