Hello There,
First congratulations for getting a 3090
I must say that I am not an expert about monitors but I want to express my opinion and I hope to help you.
You probably know this website
https://www.rtings.com/ . It is a website that it has many reviews for various products. You can try and search these two monitors and get more information. First of all the two monitors are very good! From what I have heard and seen I believe that the LG 34GN850-B is bit better than the Dell Alienware AW3420DW for gaming. I have seen some more reviews of the LG and the results were pretty positive. I am happy to view other answers and recommendations and get as well more information for these two monitors.
Thank you very much for viewing my answer