Stick with 8.1
My strong advice would be to stick with 8.1. I've been running the tech preview for some time now on a Lumia 930 because I have to and I'm developing an increasing dislike for it. Why?
It's slower - much, MUCH slower - even taking into account it's a technical preview. Slower to boot, slower to unlock, slower to launch apps, slower to take pics, you name it. Even the Lumia 950 demoed the other day is noticeably sluggish and that's able to run a Snapdragon 810 flat out.
It's oh-so-buggy. Yes, it's a technical preview. I can forgive bugs. But this is supposed to be finished in under a month. And it's not. Want to take a picture from the lock screen? Good luck with that. Want to edit it? Maybe, if you're good. Etc. Etc. Even the music app takes 5 seconds to start playing music while it wonders off into the internet to look something up first.
And don't get me started on battery life. In a pinch you can squeeze 3 days out of the Lumia 930 - it's why I bought it - but with Windows 10 it's down to 12 hours. I don't understand the rabid "IT'S EATING MY PRIVACY" response with Windows 10 - after all, Google makes its living from stealing your private data and serving you ads based upon it.
But Windows 10 on a phone is doomed to die a slow and painful death, and I'm not particularly concerned because they've lost the original vision of making it as refreshingly simple and quick to use as possible, something that was lauded in 2011.