Whether the level of certification matters depends if you care about the energy being saved. The difference between Bronze and Gold is something like 5%, so most don't pay the premium to get the slightly better efficiency.
Pricings are probably not representative of actual values right now, still in the middle of Black Friday.
I suspect the G-Series is a lower binned (ie quality) unit than the MS12 series. So it costs less despite the better efficiency.
EDIT: You definitely want a unit that is atleast 80+ certified. As the certification covers more than just efficiency of converting AC to DC current. It also covers how strict the voltage control on the rails are, 80+ spec states that it has to be within 5% of its rated voltage. If it strays outside this it can lead to the degradation of components.