Stay away from the RM 750. The secondary capacitors used in it are very bad, and known to fail easily. Also has bad heat issues that can lead to thermal shutdowns when pushed anywhere near rated capacity. Even though it does perform as stated with good output and excellent ripple, honestly with its failure rates, the CX is a better psu than the RM750. The AX is probably the best that Corsair brands, built by Seasonic, followed by the AXi, and HX, then TXv2.
For the price of those units, there are other options, Antec HCG-m is built by seasonic, HCG non-m by Delta, both excellent.