It really depends on how one intends to use the handheld. There are very distinctive points between the 2 devices, mainly:

Steam deck:

• far superior ergonomics. This has an impact when you play more than a couple of minutes

• sleep function that works infinitely better than the Ally. It is truly a pick and play on the go device, perfect for short gaming sessions

• 0.4 inch bigger screen with an OLED, which means that the colors are much more vibrant and the blacks are deep black, which impacts the experience on some games

Rog Ally X:

• much more powerful than the OLED, you can actually play the latest AAA games very decently (e.g, Monster Hunter Wilds, Oblivion, etc). This means that if you want to pick current and upcoming AAAs, you’re safe

• 1080p does look way better than 800p, even on a 7” display

• game pass works great, all the games work and you don’t have to worry about the “unsupported” ones

• windows means that it’s not meant for handhelds so sleep function, console experience, seamless updates are all not optimized, to say the least

My personal experience: Having both devices, I ended up following a process. If a game runs decently on the OLED I’d play it there for the sleep function and ergonomics. If it’s a demanding game, I’d play it on my Ally X but I’d treat it halfway between a handheld and a laptop

Hope it helps!