IF you have a graphics card capable of running it, going 4K at 144 is by far better - for everything else you may do.....
BUT, RF is not currently taking advantage of the possible step up. In RF 8, or even the DX9 version of Evolution, 2560 (QHD) was about as good as it could get.
The most current version only allows the current Windows resolution. On my 4k system, it displays fine, but no different than RF8 running at 2560.
With the change up to DX11, I expect that to eventually change. The few other games I play are stunning in full 4K So if you come across a fantastic deal for QHD AND pretty much only run RF, than that would be the current best spot. But you may be shopping again in a year or two as improvements happen and hardware drops in price.
