Well, Ferm is a pretty thorough guy so he probably knows what I'm about to kick in to the discussion already. But I'll throw it out for anyone else who runs across it.
Aluminum siding is easier to remove than fibreglass sides, that is true. Fibreglass sides are made as one sheet. To remove the fibreglass means removing the entire side of the trailer.
That being said, if buying new there is nothing wrong with a fibreglass sided trailer. They're easy to clean, don't dent, are quiet in the rain and stand up to the elements very well.
New trailers also have all those nice little bits (ie: tv, slide outs, ducted heat and ac, etc) that everybody wants these days.
Delamination can be an issue with fibreglass, but this usually shows up while the trailer is still under warranty if you know what you're looking for. The bad part is that it usually has to go back to the factory for the side replacement.
The biggest problem with fibreglass sided trailers is water leakage through the joints and seems of the roof membrane. However, if you get up there every spring and/or fall, you can dicor anything that looks like a cracked sealant or exposed joints. This does away with all the problems people moan about with fibreglass sided trailers.
Ours is fibreglass sided, we love it. Bought it new and it's getting on for 4 years old now with no serious issues. I say serious because every trailer manufacturer has problems with their new rigs. It's because of the cost cutting they use in order to maximize a thin profit margin. For example: my facets were leaking, come to find out the entire mechanism was plastic even though it looked like chrome. swapped it out with a quality Moen unit and discovered the pex line wasn't really pex. it was some "almost the same as pex" line that didn't quite fit the normal pex fittings. A quick trip back to the hardware store and pex lines are now throughout the trailer.
It seems every new RV these days are a bit of a "project" in one area or another.
But, no problems with the sides or roof. I do get up there each and every spring and check the roof though.
Cheers.