The HX850 is Platinum rated while the RM850x is Gold, however it is also 3 years newer.
They're roughly the same price. What should I get?
I'm not currently living in the US, sadge.I'm not sure what price you're looking at these for, but I suggest checking this thread out:
1200W P2 can be had for 159.99 with free shipping, and 1000W models are even less, and then you pretty much don't have to worry about the PSU for a long time, nor transient spikes and OCP on an 850W.