In terms of features, Windows 10 Mobile has definitely got a few nice things going for it. Design also looks better and more modern (which is just my subjective opinion though). Universal apps is also a nice thing. And just some other things here and there that WP 8.1 lacks.
Windows Phone 8.1, on the other hand, is far superior in terms of smoothness, stability, reliability and just general usability (no hamburger menus). Battery life is also far far better on 8.1 (at least on my Lumia 930).
It's much more than what I just listed. That's only the tip of the iceberg. Just my honest personal opinion and things that came to my mind first.
I suggest you to upgrade via the insider program and see for yourself. It W10M doesn't suit you, you can go back to 8.1 any time.