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 cannot speak for the ROG Ally, but one thing I really like about the Steam Deck is how comfortable it is to hold compared to the Switch.
also, with the switch, my hands start to feel numb after 20 to 30 minutes of playtime when I'm lying down on the couch or bed.
yeah, the switch is a nightmare to hold for anyone that has adult hands.
Not sure if you've tried this but, I've recently embraced playing with split joycons and i love it
I almost always play with split joycons if I'm docked. I recently got the Nyxi wizard controller, which is like a split GameCube wavebird, and it's way more comfortable in handheld. I wish the buttons were higher quality but otherwise I'm digging it.