The aluminum sided, so called Stick and Tin, is an older and well developed technology. It lends it self to simple box like designs and is often found on the less expensive models, because it does not molds specific to a particular model.
Fiberglass siding is the newer construction method, although it too is decades old. It lends itself to more sculpted shapes, and dazzling artwork which seem to becoming more popular.
The pros and cons are listed above. In my opinion; if the floor plan, cost and towing ability are what I want, the outside makes little difference.
However, judging from the paint jobs or decals I have seen on some units, I may be in the minority. :)