The RMi is better because it has a better fan and comes with software. But usually I just recommend the RMx because it is cheaper and is still super good. For your rig there, the RM550x is a better choice than the RM650x. You have all very efficient hardware and while gaming it shouldn't go past 225W really. Under ufll load it should be around 275W-300W.
I'd also not get those Corsair fans if you plan to mount them in a flat position. They are sleeve bearing.