I do think that getting Android 6 is important. Of all the operating systems you mentioned it is still the one with the most apps.
I know that Jolla makes it possible to use Android apps on their operating system, but not all apps seem to work.
I also dont want to buy my apps for another operating system again like I have to do now with the switch from iOS to Android.
I hope that we will see a release of Android 6 in the near future. Since Android 6 is going to officially support fingerprint readers, this would also be a good starting point for a backcover with an integrated fingerprint reader ;-).