Originally I tried to merge the latest bitcoin version into both of these. But v27 is so far away from v0.8, with thousands of git commits; I just got tied up in knots.

I tried a different approach. Forked bitcoin v27.0 and just made the bytecoin changes to that; genesis block, port numbers etc...
Found this way much easier. Hopefully moving up to v28.0 should be easy, with just a smaller number of commits to merge.

Mainnet is the original chain still of course, block 75306 at time of writing. But I never had a copy of the testnet chain, so I started a new one; block 21907 currently. It's only the testnet so I guess that doesn't matter.

So the answer to your question is neither I guess. It's based on bitcoin v27.0