Steel holds up a bit better than aluminum, but the quality beats all. Really cheap, house brand cassettes are made of aluminum cheese and wear out very quickly. High end (think XTR) cassettes with fancy alloys are made for racing, not touring. Stick with the equivalent of Shimano LX or XT steel and you'll do fine.

It's unlikely you'll wear out a crank ring in a single cross country tour, unless you maybe do it on dirt and mud. Chainrings are fairly simple to replace when they do wear out, and you can go to steel then.