Well, this is a very complicated choice if Fairphone tends to stick with only one official OS.
Coming from various “alternative” experiences (OpenMoko, Maemo), I can see the most difficult part is to let average users (read it as “not nerds nor hackers”) to be comfortable with those ecosystems and with missing apps (given that an ecosystem different than iOS and Android will end with missing “standard” apps like banking, public transport, maps, navigation, and so on) unless there is a way to permit well-known ecosystems’ apps running on the “non-standard” ecosystem.
Just an example to clarify this: on my Nokia N900 I was looking for a navigation app,
and I at first bought the Maemo version of Sygic Mobile, but Sygic ended its support
(even the maps updates!) when Maemo became unsupported from Nokia;
as an FOSS advocate I tried to get comfortable with Navit, which I consider a very well done software but it is very difficult to use for not hacker users, its UI is very poor compared to other navigation apps and difficult to use and its turn-by-turn indications are sometimes terrible and prone to errors during driving!
Ok, you can say “you’re a developer! Do your part and try to get the program better”, yes I could, but I don’t have so much time to do so and I’m very sorry for this, but the current state of the art of this project will avoid users of alternative platforms to have a simple navigation software.
If I could choose one of the options mentioned in the post, I would prefer Jolla Sailfish, because (correct me if I’m wrong) it seems the only one able to run Android apps in its environment, so users missing apps in Jolla ecosystem could find them in Android’s one.
I can’t say anything about Firefox OS but I fear it could remain a niche OS (like Maemo) only for hackers and I don’t like Ubuntu’s way of forcing user choices in a “Microsoft-like” way (like they do on desktop).
Please consider these as personal opinions and don’t open flame wars 
Bye!
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