I had a Trailmanor which has a metal exterior, (aluminum). We were close to the tornado's a couple years ago in Tennessee on our way back from Fla. The hail did $4800 damage to the top. After looking at the repair methods used I decided to get another travel trailer with a rubber roof. The insurance Co. paid me and I traded with the damage still on it.
When they build those trailers they use a vacuum platform to sandwich it all together as there is a crown in the roof. The way they are repaired is to remove everything on top, lay new metal over it and glue it down without the vacuum system.
My experience is the opposite of Jeff's - I find the fiberglass to be very eay to maintain. I haven't used a color restorer on any rig in the last 12+ years (earlier ones needed it, though). I hand apply a light surface cleaner/wax such as Protect-All or DriWash once or twice a year. Have not had to actually buff anything since the mid 90's.
But location/environment can have an effect - industrial chemicals, acid rain, etc. Jeff's Tradewinds indeed seems to get dull and needs to be re-polished periodically, while my Dolphin LX has never needed it. Something is obviously different, since both rigs are made by National RV. Mine lives in Florida about 6 months/year and "anywhere USA" the rest of the time. Jeff travels out of the West Coast and perhaps runs into much different environmental conditions??
I don't see much fundamental difference between aluminum and fiberglass from the owner's perspective. Both are somewhat susceptible to stone dings on the road, both need at least an occasional wax job to stay sharp looking, both wash pretty easily if waxed, etc. In some areas it is easier to find a repair shop for painted metal (ato body shops can do it) than fiberglass and in others it's vice versa. Aluminum siding may be somewhat easier for do-it-yourself reapairs, if you are inclined to that sort of thing, but most of us would get professional help anyway if there was major siding damage.
I'd say pick which ever "look" makes you happiest and don't waste time agonizing over siding materials.
Roofs are another story - I'd go with solid fiberglass as first choice and the shiny surface EDPM (e.g. Britek by Dicor or similar products) as second choice. Single piece metal would be my third choice but jointed metal or plain old EDPM rubber would be the far distant last choice for a roof.