R10 is in RC2 state and its great as it is but its not totally final.
Are you working in a client based couch environment ?
Then go 9.1.6
Are you sitting with it alone and have no hard deadline ?
Go R10 ... its fun to explore the new stuff and find out if your system is up for it.
Or do what I do and run both, you just need to rename the app into something else than its original name and install the other one.
Be aware that you might run into Digital Powerhouse incompatibilities which might affect RED performance