Thanks for options! What's the deal with the 9 tooth? Why not go with 10 per the XD/XDR spec? I've heard mixed review on the 9 tooth for e13 MTB cassette.
Same idea as the 10 vs 11T, a higher gear ratio (and possibly more range).
However, it's worth mentioning that while you might get the same high gear with a smaller chainring and smaller cog combo, going smaller with the chainring can also change what part of the cassette you use.
Just an example. 38-10 gives you the same high gear as 42-11. When I have a 42T chainring on, I use the center of the cassette a lot on paved sections. However, when I have the 38T on, I notice that I use the bottom of the cassette a lot on pavement, which might not be ideal long term. I haven't tried a 9T cassette yet but I imagine if I went with a 9T and a smaller chainring, i'd use the smallest cogs even more.