It's difficult, neither are perfect.
I think the Ally screen is great, and you really do notice the VRR on games, makes them play smoother. There is nothing wrong with the LCD on the Steamdeck, especially since 3.5.5 came out with the better colour settings, and the OLED should be great.. but I think I'd rather have VRR over OLED.
Using it handheld, the Steam Deck is obviously larger, but to me it's more comfortable to use, the larger size works for me. Saying that, I prefer the sticks on the Ally (A lot of people don't like them).
Can't really comment on battery as I've not used the Ally enough, at the moment for what I've been doing on battery have been no issue, but I know it's not great.
OS wise.. SteamOS all day long, every day, with 1 exception.. I wish Xbox Game Pass worked on the Steamdeck. I know there are ways to fudge it, and I could install windows on the Deck (I have done that before and it was such a pain I removed it after a few days). Having windows on the Ally means I can just use PC game pass and just install all the games I want. However, and it's a big however.. Windows is such a pain, updates for windows, updates for ROG, updates for ArmouryCrate.. seems like it's alway updating. Also you get all the windows annoyances, sometimes it doesn't wake up from sleep when you want to, sometimes it does it when you don't, sometimes things jsut crash, or the screen goes off, so many annoyances. Even if SteamOS becomes available for the Ally, then that would take away the selling point of game pass working.. I have a gaming PC so I can deal with the windows things there, I like the "console" experience of the Deck.
The performance of the Ally is better, but not that much better.. For example, I've been emulating PS3 on the Ally, and it works well, but it does also on the Deck.. games that don't play full speed on the Deck, don't play full speed on the Ally either, so minimal gain there.. AAA games for sure play better on the Ally than the Deck, but you have to tweak settings manually, with the Deck, valve do that for you (Not all ways).
In conclusion.... I will likely keep the deck and sell the Ally.. I might even sell the Deck and get an OLED deck instead.