When we were looking, we discounted pop-tops as most seemed to add 100kg of dead weight to cart around when you weren't using them.
In a PVC, unless you're going to send majority of time with 4 up, I could think of better things to do with 100kg of payload - and carry a 25Kg awning for those times when you need the extra beds
One possible advantage of a pop top on a PVC, assuming the roof cap is fibreglass, is that it should be quieter in heavy rain than the basic tin van roof.