Definitely spring for the aluminum. I had steel Torquelift tiedowns on my first two trucks, and spent lots of time trying to hammer the adapter in because the receiver rusts on the inside. Used sandpaper, files, lots of stuff to get rid of the rust in the receivers. The "aluminum" tiedowns aren't all aluminum; the part that bolts to the frame is stainless steel, the adapter is aluminum. I put them on my last few trucks, and they are maintenance free. In fact, I've used the same rear tiedowns swapping them truck to truck for my last couple trucks. I've been using the same tiedowns for 9 years or so, and they see lots of salt as I live near Buffalo, NY.
Drove to Alaska and back last year, 14,000 miles on some very bad roads with our 4800# camper and they worked great!