If your WD drives are "only" red, then you are at 5400RPM, seagate turns at 7200RPM (one of the reasons theyare so noisy).
Regarding the prices they are very similar, but if you have an opportunity on sg, why not.
Personnally, I do not understand the purpose of "pro" raid disk... it is anachronic.
Even if the brands can be mixed together, the tech specs must add up too.
Anyways, the rule of the slowest is the king applies, whatever you have as pro, plus, gold, platinum, enterprise, exos... it will be the slowest who will gives the tempo.
I am ashamed, I had never heard about SMR/CMR :s