For games I tend to think that the Radeons offer better value. However that article above does seem to suugest that NVidia cards offer lower rending times when doing GPU rendering (or combined cpu and gpu rendering). I looked at this before for my daughter a few years ago (when she was making youtube videos with adobe premier pro etc) - despite me saying that it appears NVidia cards may be faster my daughter currently uses a vega 56 as rendering is only a proportion of her computers usage (she plays more games than doing her uni work! computer games modelling and animation degree).
I attach blenders system requirements that doesn't seem to favour NVidia over radeon
https://www.blender.org/download/requirements/
I also attach adobe premier pro's system requirements which also doesn't seem to favour NVidia over radeon
https://helpx.adobe.com/premiere-pro/system-requirements.html
As you can use either graphics card then I tend to think that you can get a more powerful radeon card (overall but not necessarily more powerful in blender) for the same money. With some of this software it depends on how the software is configured (ie. cpu or gpu or cpu + gpu rendering) and I seem to remembering looking something up for a similar software to blender a while ago that worked better if you installed a CUDA app for NVidia cards (but if you hadn't installed that app then NVidia cards wouldn't be much better).
If rendering will be the primary workload for our computer then perhaps you should be looking at £350 for a graphics card (for this price you could get a NVidia 2060 super or a radeon 5700 or even a 5700xt). Generally for gaming a mid to high res (1440 or 4k) the 5700xt would be superior to the 2060 super but that review above shows the 2060super to be better in blender.
having a fixed budget somethings got to give - if you get the beast of a cpu the 3900x then you get a lesser gpu. if you get a better gpu like the 2060 super then perhaps you will have to settle for a 3700x (which will still be very good). one consideration that would bother me is the upgradeability - it is easier to swap out graphics cards than the cpus (and as microsoft windows links the licence to the motherboard and cpu much more strictly nowadays [in August I tried to transfer a ssd with windows on it to a new mobo & cpu it noticed and forced me to get a new licence and similarly on another pc]. Overall though for blender a slightly lesser cpu or gpu will only cause you to wait a few more seconds probably. A few years ago my daughter was making youtube videos and transcoding using handbrake (after creating in adobe premier pro) to make sure that the video and audio were aligned all the way through at 1080 to being 30fps and it took 30minutes to many hours using a quad core phenom - nowadays using a ryzen 1700 it breezes through (and a 3rd gen ryzen would be much more powerful - roughly double).