It's a personal preference with regard to looks. People have already mentioned the washing and waxing... Definitely seems easier with a smooth-sided, fiberglass trailer... Less surface area and nooks and crannies.
One thing I have heard that aluminum is easier to fix if damaged... Just pull out the damaged panel(s) and slide new ones in... Probably not quite that easy, but thats the way it was described to me. Fiberglass repair is a little more involved and probably specialized.
When I was looking for my last trailer, I did consider aluminum for a bit but found that the models I was looking at we're heavier than the fiberglass equivalent in the double over double floor plan. Might not be true across the board, but was true forthe models I considered.
Again, personal preference. There is a lot of discussion on these boards about the draining of aluminum (often times wood) vs the aluminum framing that is typical in fiberglass-clad trailers... Definitely camps that prefer one over the other and they all offer compelling arguments for their preference.
I would just suggest you decide on a floor plan and start looking at models, comparing weight and features. If you really prefer the look of one over the other, then it's probably wise to go with that, all other things being reasonably equal. If you don't, you may always wish you went with the other... ;)
Good luck.