While Jolla will most likely have a much better and polished UI than Ubuntu, I'm not convinced Jolla can get a big enough user base to create good user- and third-party-support. While Ubuntu will most likely have a horrendous phone experience in the beginning, giving the fact that they are trying to squeeze desktop applications in a phone (very maemo style imho), Ubuntu is a very well established brand. Even if Ubuntu can't really compete from a user experience perspective at first, I think they will continue the development of Ubuntu Phone for a long time to come. I'm not convinced Jolla will have that luxury.
Tizen seems a big mess, and I don't see anyone that is willing to clean it up properly. Samsung is way to much Android oriented and I'm not convinced Samsung or Intel are even capable to fix the OS and release a good product.
Firefox OS is intended for html-only application, so that isn't an option for me.
For me any linux distribution needs to have an active userbase, otherwise it is way to much work to update and support stuff myself.