Ubuntu but I'd change for sleep.
I'd guess that the X13s hardware support in Ubuntu is likely as good as Debian, and switching probably wouldn't help you much. I have noticed that newer kernel versions (notably 6.12 and later) and the latest firmware (as of sometime last year) really fixed a lot of little issues for the X13s. That probably makes a bigger difference than the distro. I'd check to see which versions you're using.
Personally I'd still just get a used ThinkPad X13s Gen 1 on eBay at this point if I were going to buy another one today because they're available for around $400, and I don't see anything better that's passively cooled, with great battery life, and great Linux support available at the moment. I hope there will be a better, faster alternative in the near future. I'd gladly upgrade.