Since you currently are only using two components that are DA, I would say use the 7900 RD and sell off the 7900 shifters and FD. As someone mentioned, the 7900 RD can take a larger cog than 7800, though it is not as attractive IMO. I didn't think the 7900 shifters I had were anything special and they died sooner than I thought they should have. I actually went back to 6600 shifters so the bike is a mongrel of 6600, 7800 and 7900.