If my memory serves me correctly i'll try and give what I know.
George Gardner of GAP developed the 6.5 SAUM 4s it was designed to be the ultimate short action cartridge as well as offer optimal ballistics for the PRS series which has a velocity cap of 3200ft/sec I believe. Part of the design philosophy was to run long barrels with slow powders(H1000) to get the velocity rather than wringing the performance out by high pressures which ultimately attributes to excellent barrel life in proportion to the performance delivered. The one caveat of the 4s is currently hornady is the only brass available (some form brass from 7 saum norma brass and ect...)
The 6.5 PRC to my understanding was a Hornady/Ruger project that George of GAP helped work on. it too is aimed at the PRS market and is built with similar design principles of the 4s but uses the RCM (ruger compact magnum) as it's parent case rather than the SAUM. The 4s has slightly more capacity over the RCM.
For general purposes there so similar you could likely flip a coin and choose one. The big difference that I see is the RCM is backed by Ruger and Hornady so it could potentially go further in the market just due to the backing of such a giants. don't get me wrong the 4s has it's followers but most who run the 4s have to coin to spring for the best. the PRC could bring a new crowd into the ever growing sport especially when off the shelf guns like Ruger RPR hit the market.
