But you don't want to play that long on a portable device anyway, position is just terrible for your posture/neck and I don't think it is really healthy. What you want is alterning occupations: video games, reading, sleeping, drawing, card games, listening to music, making music, chatting, eating, moving in the main aisle, etc.
The ally is much less efficient with the same battery capacity. I've seen reports of like 40 minutes of battery life when running high power draw games.
But would you want to spend that time playing with an undocked 700gram device in your hand, in a bad posture looking down at a screen? Even my much lighter Switch I would not play more than 45 minutes straight in portable mode.
The ally just seems to be chasing 'look how high the specs can go' instead of practically improving on the steam deck.
Brief summary - more recent APU so if you're plugged in to power, the Ally can play games at higher settings. If you're not plugged in, reviewers found that the minimum power draw is high so your battery life is worse (both have the same battery capacity). The best case scenario for the Ally is 4 hours while it's closer to 7 for the Deck.
On the software side, the Deck benefits from a large community of folks creating control mappings for older games. Windows handicaps the Ally - it's just clunky to use in this form factor.