the RC is the recicrculating type.I think Repto has experimented with that and could comment on its effectiveness.One of the benefits of the RC type is that water can be fed straight from the tank overflow into the skimmer.This is the most effective way for removing organics via skimming.Traditional skimmers intake water from the sump usually near the bottom.Often,due to poor sump design,the proteins have already migrated to the surface and will never reach the skimmer pump intake.
That said,I like the simple,cheaper design of the non-recirculating models such as the RS8-2 and the CS series.The above mentioned problem about surface migration of organics can be helped by proper sump design.
With the CS series,your paying for higher quality acrylic/craftmanship although i also feel the RS series is high quality(It is the same series i have)Being that they both employ the same design(needle wheel) i would think that performance should be close to the same.