Can you explain how you measure which one is more mature and polish?
I have owned both phones for a while now and I ran FFOS on my now long dead Nexus 4. Both the Jolla and Aquaris are "stable" in the sense that they rarely crash. In terms of user experience, the Jolla is vastly superior; the OS is much, much smoother to navigate, there are many more applications available for it and you have the option of running Android applications too which do actually run rather well. The Ubuntu phone is in all honesty just about usable in terms of lag. Opening something simple like the system settings takes a noticably long time, and this is with nothing else already open. It just isn't snappy enough to be used as a daily driver, in my humble opinion. That said, I like the screen and the battery life seems superior to the Jolla. I also like that the Aquaris is open source, in the true sense of the word. Both are superior to FireFox OS which I thought never really got going, which is a shame as I loved the concept. In terms of alternatives, if you find any please let me know.