I mention this in the last section on recommendations, but the other components of a trailer as just as important as to whether you are looking at steel or aluminum. The grade of the plywood deck is critical. Is it a 7-ply marine grade plywood or just half inch exterior plywood that is painted. The trailer my look good, but closer inpection can reveal that the quality if very poor. Even if your not an expert, you'll notice the lack of quality materials if you spend the time to look.
Trailer covers and enclosed trailer options.
Many snowmobiler's start with an open trailer and upgrade to after-market covers and enclosures at a later time. This has several advantage and disadvantages but in most cases, more advantages than disadvantages. There are no less than twenty enclosure manufactures that produce high quality enclosures for 10', 12' and 14' trailers. I know of a few that even make enclosures for 24' and 26' open trailers. The construction materials range from a nylon fabric, which is not worth buying, to high quality aluminum and fiberglass.