I would be suprised if the "plastic" rotted, because gliders have been made of fibreglass for decades.
The main difference is likely to be that removable fibreglass parts are going to see more damage from routine maintenance; the biggest source of widespread but minor damage to aircraft by far. It's very easy to crack a fibreglass cowling by monkey handling - £5000 gone just like that - and I've got the same problem on the TB20.
On the Cirruses and Diamonds, I am advised that there are a lot of internal parts e.g. around the engine which rot. These are stuff like zinc plated steel.
I think aluminium airframes don't start rotting for something like 15-20 years, as far as the major parts go, but a lot of smaller parts start well before that and the "plastic" planes share those parts too.
One might wonder which will last longer parked in Spain. I don't know.