I have a Lund sitting on a galvanized trailer. It's a 2004 model, not a speck of rust. The galvanized gets a little tarnished over the years, but really has held up damn well for me.
Let's put it this way, until they start making the trailers out of stainless steel, I would spend the extra couple of dollars to get galvanized. I have had to replace the spindles and hubs as they were pretty wore out (seals no longer held water out due to grooves in the spindles), but the frame is going nowhere.
I do need to replace the painted guide bunks. Wish they would have been galvanized as well.