If you have the money I would get the 3090. 3080 is good for 4k as well but I mean. Regardless of the % increase, the 3090 is going to run better than the 3080.
Allegedly. At 4k, that is.
I myself have a 2k monitor thats going to become my 2nd monitor when I get a main 4k one. That will be after I can get my hands on one of the Asus ROG strix 3080 or 3090s. Whichever I can order/ whatever is first available is what I'm gonna order.
I don't see myself gaming past 4k resolutions so either card is gonna work with me, since at the moment money isn't too tight.