I have stayed at both, and I prefer the pool at CSR, but prefer the overall "old south-Gone with The Wind" theming at PO-Riverside (which was called Dixie Landings when we stayed there back in 1998.)
I thought the transportation was better and quicker at CSR, but PO-R has the boat to Downtown Disney (doesn't it?--I know at one time it was suspended.)
If I had to choose today, I'd say CSR. Cabana 8A is a building that sits sideways, so you don't get any direct parking lot views. I'd also go with the same Cabana again. If 8A is not available, then Cabana 9B.