The AR-15 cheek riser goes further forward/closer the receiver than the more abbreviated AR-10 version. It is nice to have the addt'l length on the riser depending on how you'll be setting up the rifle in terms of your optics as well as how you personally fit the rifle. I shoot closer to the CH on my ARs for the most part and the shorter AR-10 cheek riser can be a pain at times in terms of being partially on/off of it if you get too far forward. If it were mine, I'd opt for the AR-15 version PRS since you don't have the clearance/functional limitations stemming from the longer, AR-10/SR-25/LR-308 charging handle on the REPR given its side CH.