The Fitbit is a little more flexible, looks cuter and has “moar features” in terms of tracking your climb etc. However it won’t sync with my phone and there’s no viable Linux sync solution AFAICT which would lead to having to have a CackOS box or a Wintendo (and yes, you can draw whatever conclusions you like about my OS preferences from that).
The Jawbone however syncs with Android phones (and mine is on the supported list) via the headphone socket which is quite clever and plausibly even reverse-engineerable. But the Up is a tad more expensive than the Flex and not as convenient in terms of wrist sizing.
I would be very interested to hear from any Flex owners or Up owners, particularly about their experiences using their devices where there’s no Apple or Microsoft involved. I am aware that some previous Fitbit products worked with Linux (fitbitd) but none of the sleep-tracking ones seem to.
For reference, I run Debian/Wheezy (although I am persuading myself to switch to Jessie soon) and my phone is a Galaxy Nexus, running stock Android (although it is in unlocked-bootloader mode).
Please supply ideas.