I have bikes with Rival and Ultegra 6600 and 6510 and like SRAM and Shimano equally. I favor SRAM brakesets though, especially the newer Force and Red but even the Rival is good. Assuming you are sure a tripple is not in your future, I recommend you take test rides and pick the one you like. I understand the new 6700 is not perfectly 100% backward compatible.