If you feel like you HAVE to get a set of tire covers, go with the parachute-style breathable (nylon?) stretchy fabric. They will block UV rays (which is what damages tires over time, in theory) but let water and air flow through. The criticism of the leather (or heavier material) covers is that they actually trap IN moisture and heat, which can contribute to shorter rubber life.
I got one of the above-mentioned styles for the spare tire on my TT, just so it wasn't hanging out there in the open all the time. It's white and matches the trailer, so it looks decent. Cost something like $10 on Ebay I think.