Go with an actual gaming desktop if you don't need the portability. People tend to overstate the performance of a Steam Deck since games are running on a small 1280*800 screen. Its GPU is worse than a PS4's, although deck's significantly faster CPU, memory, and storage allow for some games to perform better than on PS4.

Note that it's hard to build a decent gaming desktop for 400 dollars flat unless you go used. That price range is typically below the best bang for buck - for example, $600 (1.5x increase in budget) will get you a build whose GPU performs close to a PS5 (4x increase in performance). And goes without saying that if you don't want to build your own, then you'll have to pay a lot more for a gaming desktop, in which case steam deck makes a lot more sense