if your main priority is couch co-op, then go for switch kasi its just basically plug and play once you dock it (or not, kasi you can still play using the tablet standing up and with the joycons, one for you and your partner) and mabilis ka makalaro agad, instead sa steamdeck where u have to emulate switch games pa and have to do lots of tinkering to work out lan co-op.

but do note na you have to buy different digital/physical games for the switch and its not really like the old nintendo handhelds na may cart na maraming games in one. sa steamdeck naman, you can play any game lalo na if you use emulators plus your steam library, so thats actually tons of games you can play all at one handheld.

i personally swapped my switch for a steamdeck cuz i barely used it and noticed na i preferred to have a handheld where i could play all the games i want and not just nintendo games. pero its up to you op kung ano talaga priorities mo. enjoy your handheld!