I've not heard the BM9, but I know Sam was very impressed when he reviewed it:https://www.soundonsound.com/reviews/extinct-audio-bm9
It also has the advantage of having a significantly stronger (+12dB-ish) output level than the Coles 4038, which makes a big difference to how much preamp gain you need.
Of course, the 4038 is a lovely mic too, but physically larger, more fragile, more expensive, much lower output, and rather harder to mount!
However, I would just question the appropriateness of using one or more fig-8 ribbons on a piano. It can and is done, of course... but placement can be tricky and room acoustics can play a complicated role in the overall sound.
I'd strongly recommend hiring and trying before buying.