They definitely each have their own strengths and weaknesses, if I had to pick a winner, I’m going with Steam Deck, you just can’t compete with their controller API and the Deck’s assortment of input options; that alone makes everything else obsolete.
Raw horsepower(Rog v Deck) is a meaningless metric; Using the Rog is like driving a 911 without tires, just rims; you literally can’t control it.
I know this literally contradicts your last sentence, but I got Diablo IV up and running on desktop Linux really easily and it runs great. I followed these instructions minus the ProtonUp-Qt part (Proton Experimental runs it): https://www.gamingonlinux.com/2023/03/heres-how-to-run-diablo-4-on-steam-deck/
It amounts to: