Breatheable vs not
Rafters perspective: Training a new guide up in AK on the Nenana I got to see the real value of Gore-tex compared to non-breathable (I call them bag) suits. It was a beautiful day for AK, about 65-70 and sunny. The trainee guide is working hard to keep the 18' raft in the right part of the current and make the relatively easy class II moves. He is sweating. About half an hour later he open the right wrist gasket of his bag - and sweat literally pours out. He repeats this with the other wrist and both ankle gaskets. During the next hour this gets repeated two more times. Bag suits and drytops are really like wearing a high tech trash bag - they keep outside water out and inside water, ie sweat, in.
When you are working hard in moderate temperatures you sweat a fair bit, and gore-tex really does breathe. For a rafter who gets less wet than a kayaker the breathable fabric makes a real difference. That said a non-breathable drysuit is a heck of a lot better than no drysuit, and in really cold conditions with moderate or low work levels the benefit of the breatheability is minimized.
It is also worth noting that if you mostly do 1/2 day to single day trips the side effects of being damp from sweat aren't huge, but your odds of ending up chilled at the end of the day increase. If you are doing a multi-day trip in cold conditions the benefit of keeping your insulating layers dry and effective can be huge.