CX picture is slightly better, deals with dirties sources better, will have an extra year of LG software updates, has many AI picture options which you probably won't use, its motion is better (even without BFI) and its picture is slightly better again.
The C9 has full 48gbps ports, more apps in the UK. It has been reported slightly brighter but IMO its panel variation. Its also better value.
HOWEVER... just as a bit of friendly advice from a home cinema and PC enthusiast...
In real world terms, I think the you will not miss anything from the CX but it is a marginally better TV PQ quality wise. I'd go for C9. Well, actually I'd go for whichever TV is sold by John Lewis if you're using it for the PC and get the protect plus warranty/insurance. Features like DTS, 48gbps, AI picture options, motion, upscaling quality are meh IMO but Burn-in is and always will be the enemy for users like us who PC game. I take it you are the same as me.. in that you will connect this baby to your PC.. and maybe sometimes you will forget its an OLED and browse the web on it... etc. etc.
John Lewis offer a protect plus warranty which will offer for £150 replacement of TV/panel for burn in for 5 years. That means you can actually use your screen for anything and not have to worry or do any workarounds. I find that peace of mind absolutely invaluable. Its even better than Best buys geek squad warranty.
Funny that. I bought a 65” C9 yesterday from John Lewis with the £140 warranty for exactly that, peace of mind.
At least for 5 years I won’t have to worry about what I use it for.