There's not a game I prefer on the Switch over the Deck but I do think both systems have their merits. I've played through Zelda TotK on Switch recently and play my Deck more often (lately a lot of BG3)

The Deck is honking huge, while the switch is small. I much prefer the size of the Deck to the Switch but there's a an easy argument to be made there for portability.

The Switch is just more dock-friendly. Any serious Switch owner (who would care about docking) owns a Pro Controller. Docking to and undocking from a TV just works perfectly 100% of the time. You can buy a dock for the Steam Deck and you could Bluetooth a controller but it's definitely more finicky - specifically with some games especially.

Multi-person household. I have a wife and a kid. I'm very fortunate and my wife has her own Deck but we do share a Switch and a PS5. There were dozens of times I would have played Zelda or FF16 but my wife was using the console so I settled with my Deck. I could easily see the opposite happening if we didn't each own our own Steamdeck

Ring Fit Adventure :)

Basically any indies or games that have been ported to Switch, you're gonna get better performance on the Deck.

There are a few games like Okami which do neat things with the joycons and go on sale often enough that I can justify buying them on the Switch.

I still get the occasional Nintendo exclusive on my Switch for the sake of convenience. I'm too lazy to try and emulate TOTK on the Deck for example.

Final Fantasy VII is probably the only game I’d rather play on Switch, not counting Nintendo exclusives that I can’t be bothered to emulate. The Switch port is the updated version that’s available for windows through the Xbox store, but for some baffling reason the Steam version is older, lacks the new accessibility features, requires a launcher, and just all around does not want to work well on modern computers.

I have an older switch vulnerable to fusee-gelee, so I've been using yuzu's tutorial for how to legally rip my purchased games from it.

I only use my Switch now for 1) Nintendo exclusives, that 2) I've already purchased, and 3) don't run well on Yuzu. So... Super Beat Sports, mostly. (Harmonix please make a PC port!)

Switch is an absolute last resort. It only comes out when an exclusive drops, and the exclusives are only played if the switch can actually run them properly.

I haven’t played TOTK yet because of the performance woes.

If there are no notable differences (like Undertale, Baba Is You, etc.), whatever is cheaper at the time. Otherwise Switch is Nintendo exclusives and SD is other stuff.

Maybe if I can't emulate a switch game well. But realistically I'll probably just not play it. The switch isn't actually meaningfully smaller once you put controllers that don't make my hands want to be amputated on it. And even then the inputs (especially sticks) are way lower quality than the steam deck.

Sometimes I can use Gold Points to get a game on the Switch eShop for dirt cheap. That's the only time I'll buy a cross-platform game on Switch. Otherwise I only use it for console exclusives now.

Whenever possible, all of them. The switch may be underpowered, but for the majority of games it doesn’t matter. The deck feels unpleasantly oversized, like the original original Xbox controller or the game gear, and awareness of it’s presence doesn’t fade into the background no matter how long I’m playing. The switch is a joy in comparison.

Whatever I own on the Switch that I don't want to (or can't) buy again on the Deck.

I gave my switch to my SO. I had forgotten that I did.

Mario Kart cause local multiplayer

I think that's it

If I already own the game on either platform, that's where I'll play it. I also don't emulate the Switch on Deck, as (and although) I have a modded V1 that I can OC and a Lite for portability's sake.
I especially prefer playing TATE Mode shmups on Switch with my Flip Grip, it's a game changer.
I also tend to favour physical games overall, so I might buy games on Switch more often than not (unless there are significant performance differences).

Super Smash Bros Ultimate. How else am I supposed to end friendships?

TOTK on the Steam Deck has been a much better experience than natively on the switch.

Also bad switch ports (mostly because it's just too underpowered) like The Outer Worlds run great on the deck

Switch exclusives only for me... But I'm an absolute fan of Xenoblade so I cherish my switch and refuse to emulate so I can show Nintendo with the time spent on the game that I love them ^^. But since both Xbox and Playstation are making PC port, when a PC port of the Xenoblade trilogy, Nintendo? 😭

I own both and my wife only has a switch.

I play any Nintendo exclusives or indie games we are going to play in parallel on the switch. Anything that I will enjoy in handheld other than that I play on the Deck.

I purchase and play switch exclusives on switch. Multi platform titles I buy for the deck. I don't really pirate much these days.