I have had one of each in 508. I reguarly weigh my boards so I can tell how much water they might be taking on if i ding them. Also good for early warning with things like little cracks around the fin boxes that you would not otherwise notice.
I think each individual board may be different, but here is what my ones weighed.(weight includes tail pad)
Sweet Potato FST 508. No Fins, No wax 3.777.
Sweet Potato RF 508. No Fins, No wax 3.267
I owned a RF for 2 months, then replaced it with an FST, then went back to RF as soon as possible ( 1 month) due to lack of performance in smaller waves. For me The FST went ok in bigger waves but was majorly lacking in accelaration and general livelyness in smaller, Knee to waist, high waves. When I got my RF back it was everything I expected.
My other boards are FST and I am completely happy with them. For me the potato is about small surf and I think the extra weight of the FST potato that I owned is not good in this area.