IMHO it's not the trailer frame that get's fed up the most it's all the axles and brakes and stuff. Most high end trailer makers will put a flush kit on so you can get all the salt out of the hard to reach places.
Yes a alum. trailer wont rust but if you don't take care of either one it's going to need a lot of work in 5 years. I think Myco and Extreme build alum trailers that look close to their steel ones.