look it's like when buying a car.
you can get some chinese brand car that got 150HP or you can get a nice BMW with 150HP
both advertise on 150HP
but HP ain't everything.
the BMW will use higher quality parts, will be of a higher build quality, will have better accelaration & breaks and will have all the saftey standards you need while the cheaper chinese car won't
it's the same here.
the RMx will bring the advertised wattage @ the 12V rail, which is what matters
it uses higher quality parts that ensure that the PSU will run for many years to come even if it gets hotter inside your case
it ensures that if - god forbid - something goes wrong when you OC or something goes wrong in your electrical circuit due to thunder and poor electrial work in your house, the PSU and more importantly all other parts in your case will remain safe.
but as basroil said, high quality Corsair PSU don't come cheap, EVGA, Seasonic & XFX are probably your best deal.
I'm using my XFX PSU for many years now without any problems