Asus ROG Ally has the better hardware.
I've got a 64GB Steam Deck with a 512GB SD card and my brother's got an Asus ROG Ally.
I really like the Deck, but when it's ran out of oomph, it's got nothing to give. The ROG Ally can be used at 15W for better performance than the deck (Can be tweaked a bit more too) and then when you need to you've got another 15W you can use.
Microsoft needs to cater their OS more towards these new handhelds though, as Steam OS is a lot better at ease of use.
Ease of use is a massive point. I tried the Ally and lack of a trackpad is a deal breaker. Deck has 2.
The control customisation and community layouts on the Deck really make it for me.
It also has Proton so can run older games that even the Ally cannot natively - Motocross Madness 2 for example.
I would get the Ally if it was being used for a specific AAA title because it does have a better cpu & gpu, however that extra power is crippled by the battery life.
What’s Asus support like? I raised a ticket with Valve for my 2nd hand Deck (not booting from usb stick to recovery) they suggested RMA and sent me a brand new one!
What’s upgradeability like with the Ally? It was easy to swap my 256gb drive for a 2tb one on the Deck. There’s also Valve teardown videos and many many deck upgrade options.
More to think about that just CPU & GPU.
The screen is better, i’ll admit that. The Deck screen isnt terrible but its not perfect - looks good 1280 x 800, 7”