If you have NO handheld, get Steam Deck. PERIOD. There is no contest here. You only buy Switch if you want 1st party titles and their online functionality.
Example: I want to play Nintendo Switch Sports online. I HAVE to buy a Switch to do that (in a nutshell). YES, I can emulate the single-player on Yuzu/Ryujinx emulators, but no online.
With Steam Deck you'll get potential access to pretty much any game store you'll care about (Steam, Epic, GOG, etc). You'll get cheaper game prices through Steam Sales and game keys. You'll get an entire PC with it. And so much more.
As long as you keep your graphical and performance expectations in check the Steam Deck will offer you way more in the long run.