The P9 is powered by Marshmallow (android 6.0) and the UI is EMUI 4.1.
The P9 is indeed very fast and responsive not only in terms of picture capture but also in apps launching.
Compared to the P8, the P9 has more advance photography features which makes it ideal solution for night shoots with less noise.
In fact, with the P8 upgraded to the latest version firmware B321, the manual photography options are are similar to that of the P9.
If you already own a P8 bought last year and is still functioning well, I don’t see any reason to purchase the P9 just for the Leica brand.