Chary is 100% correct, both is the right answer regardless of the point of this poll ;O;. You get the best of both worlds, your latest Zorldos, Murios, and pogermins with the Switch and then basically everything else with the Steam Deck.
But if I was forced to have just one, Steam Deck for sure. You get a much better value for your money at this point, with a few thousand officially playable PC games, and the umpteen thousand titles you can emulate basically up to Wii era and potentially above depending on the game. And given it's just a PC, you can do whatever you want with it software wise without having to worry about bans or lawyers being sent after you.
Steam Deck by a long margin - as it's also a Switch. My 7 y/o has taken my 512 Deck hostage and has promised to give it back once he's been through Kirby and the Forgotten Land, which looks fantastic. If you're reading, hurry up, kiddo. I want to go back to playing Handhelden Ring!
You just install/run Emudeck and it does everything for you. I have plenty PSP games playing in there.
But that said, in case you want it so, plenty games from PSP era can be bought and played on the Deck.
Some of them may give you a problem or two (e.g. Trials in the Sky video intro lags and stutters) but that can be solved easily (ffmpeg recompress the video to 30FPS 1280x600).
Also you can install, add, and mod games, and plenty more. It is not needed but it is part of the fun. I believe this makes it extra interesting for people that like to tinker, and that used to be gbatemp's target anyway.
For me it's an easy choice:steam deck all the way. Heck... It can even emulate switch games...adequate(haven't fully tested it. Only played through links awakening without issue).
The selection of games is simply beyond comparison, and even keyboard/mouse games are doable (amongst others, I'm now playing through unreal again).
The thing is... I can only really speak for me. I've got a giganormous steam library, don't care for console exclusives, and don't mind tinkering to get things to work. Oh, and I've got large hands. I've held a switch only shortly, but I don't think it'll be as great to hold.
Again: for me. I imagine people with smaller hands(children) have it better. Their lineup isn't as big but still very respectful in every way. Okay: and that it's cheaper and more available are factors in its favor.
Bought a used Nerf Mega blaster yesterday, good condition, but only got one single foam dart for it. And it was so chewed up, when I tried to fire it today it ripped the tip clean off and spat the thing out in two pieces.