Battery life and SteamOS.

Yea the Ally can beat the steam deck in performance, but the battery life will be awful. If you want to get the same battery life as the steam deck you'll get about the same performance.

Also steamOS is a much better OS for handhelds than windows. SteamOS is just more optimized for a handheld, it's snappy, doesn't eat all your battery in sleep and can play most windows games, sometimes even with slightly better performance.