Re: advice please
There's pros to both sides. 8.1 is obviously the more stable OS, but also if you jump into Win10 mobile now you have that time to actually get used to it. Especially for someone who is just getting on board with Windows Phone... getting used to a brand new UI just to upgrade in a few weeks/months to a similar, but still quite new experience might be a little annoying.
To be honest, I would jump in with Win10 build, especially considering the is a new build on the horizon promising to be even more stable. I actually did the same thing and convinced my partner to jump on the Windows Phone bandwagon from android. She got a 640xl and I promptly put Win10 10166 on it. She's encountered a few issues here and there, mostly with the photos app being somewhat buggy. But overall she's enjoying it.