There are three NUC13 Pro, with i3, i5, and i7. The single thread rating doesn’t differ between them. This is the most important thing for Roon UI and database performance. The i5 and i7 have more cores, which helps with playing to many simultaneous Roon zones (like if you live in a castle), some very heavy-duty DSP (though an i3 should suffice for anything reasonable), and a bit with search. The effect of cache size depends a lot on the specific software, I doubt that it does much for Roon.
Therefore, you should be fine with an i3 for most uses. 8 GB RAM for up to 100k tracks, 16 should be more than you will ever need.
From: Intel i3-1315U vs i5-1340P vs i7-1360P vs Apple M4 Pro 12 [cpubenchmark.net] by PassMark Software
As you can see, the M4 is faster in single thread performance. (Though it’s not as if a NUC13 is slow).
The smallest SDD in the M4 is more than enough for Roon and its database. If you don’t mind the cable, for local music you can attach an external SSD for much less money than the internal 1TB.
You should be just fine with 16 GB RAM unless your library size is enormous (like 500k+ tracks or so). The RAM size is less critical on macOS because it swaps, so you will never run out of RAM and just crash like the Roon OS in ROCK does. And the SSD is fast, so swapping to disk doesn’t incur a big speed penalty.
I think it mostly comes down to what you are most comfortable with. You can run other stuff on the Mac (like Roon extensions, HQPlayer or entirely different programs). You need to change some settings and do some (minimal) maintenance on the Mac. macOS isn’t very annoying IMO but as we have seen in the many recent threads about macOS 15, there is potential for complications with Roon after OS updates.
The ROCK takes care of itself more or less but you can only run Roon and have no insight into activity and performance metrics. (On the other hand, I never needed to have it on my ROCK)