It'll be mainly bag, but sometimes heavier stuff due to having the rack for those duties in the first place. The wald racks suffice for the lighter duties.
I went steel for my commuter because there's the odd time I'll load it up with beers that take it close to the weight limit - I don't want to fuck around with something exploding into my front wheel!
This is correct. If you're going to be carrying medium to heavy loads semi regularly on anything past tarmac, steel is the way forward.
If you're carrying a light bag for commuting, then alu will be the lighter choice.