I am currently using GNOME and that’s the best DE I’ve ever used it. It’s polish and stable. The apps look great and it’s doesn’t feel harder to use, it’s easy! And some extensions are great like OpenBar (to make your top bar look fancy).
I’ve tested KDE as my daily driver for a day and the experience isn’t bad. It’s just that there’s too many settings lol. I know that’s one of the strengths of KDE but for myself I can’t stand. I want a simple experience OOTB so that’s why.
In terms of security, I have heard GNOME is safer but I’m not an expert on this topic so yeah…
What I could definitely say though is that these 2 DEs are great to use. Test them first and see what you like.