As long as you don't leave it out for long periods of time in a wet environment, a steel frame should serve you well. Take care of it, and it can last for a lifetime. A little bit of rain isn't going to make it rust. Besides, should a catastrophe occur, a steel frame can be repaired, while an aluminum frame will be toast. Most touring bikes being sold will have steel frames.
Don't worry about weight--you'll be loading down the bike anyway, so you'll want a bike that can take it.
Still, the most important thing is having a bike that fits you, and which you'll enjoy riding for long distances.
A page to take a look at is this one--
http://www.crazyguyonabike.com/doc/page/?page_id=8364
It's written by a guy that's done a lot of touring. That page is his section on frame materials, but the rest of the site is well worth reading.