The heavier the trailer, the more fuel required to draw it around. Aluminum is as you know, lighter than steel, but you need to heed the areas where the aluminum and steel meet. The suspension to trailer area and every bolt are the main problem areas, especially if snow melting agents come into contact with the trailer. These areas are prone to corrosion from the dissimilar metals issues. On the other hand, the bonus of aluminum is it doesn't require paint. Aluminum is typically more expensive than steel, but in my one opinion, would be the way I would go if I could afford the extra $ for aluminum.
www.craigslist.org may have some good deals on used trailers.