Fitbit

One has an altimeter, a sleep tracker and is prone to getting lost easily. Flex is more convenient but without those features. It's also slightly less accurate.

Flex can also be worn while swimming, showering, etc. and the Flex does track sleep just like the One.

The biggest advantage with the Flex is that, in theory, you never have to take it off. With the One, you can't get it wet, and you have to move it into a special sleep band if you want to track sleep.

You also have to remember to keep unpin the One or take it out of your pocket before you throw your drawers into the laundry. ;-)

The biggest disadvantage with the Flex is probably that hip-worn pedometers are generally more accurate. The Flex will have some minor issues with under-counting if you walk while holding your arm still (e.g. holding onto a bag), and can have some more serious issues if you do a lot moving your hands while sitting. Fitbit tries to take those things into account but, from what I've seen, Flex is still less accurate than One.

I just love my little zip.if you just want to track steps I would go zip. if you also want to track sleep go one.

I have just weighed up the relative benefits of One and Flex and read all the reviews on Amazon UK, as a result of which I've just ordered a One. Lack of a screen and longevity issues with the Flex band were my main reasons for going for the One.

I heart my Flex! Easy to use, never lose it, looks like a sporty bracelet and the alarm is awesome.

I prefer the one that's like a bracelet - the one you put on your bra or belt would never work for me as I would always forget to put it on.

One has an altimeter, a sleep tracker and is prone to getting lost easily. Flex is more convenient but without those features. It's also slightly less accurate.

Flex can also be worn while swimming, showering, etc. and the Flex does track sleep just like the One.

The biggest advantage with the Flex is that, in theory, you never have to take it off. With the One, you can't get it wet, and you have to move it into a special sleep band if you want to track sleep.

You also have to remember to keep unpin the One or take it out of your pocket before you throw your drawers into the laundry. ;-)

The biggest disadvantage with the Flex is probably that hip-worn pedometers are generally more accurate. The Flex will have some minor issues with under-counting if you walk while holding your arm still (e.g. holding onto a bag), and can have some more serious issues if you do a lot moving your hands while sitting. Fitbit tries to take those things into account but, from what I've seen, Flex is still less accurate than One.

This.

I have the Flex and am glad I have it instead of the One. Most of the time I forget I even have the Flex on because it's so comfortable sitting there behind my watch. I always use it to track my sleep and wear it in the shower. I only take it off to charge the battery. But it does have it's down sides. It undercounts my steps unless I remember to swing my arm, which means I try not to carry things in that hand. Also, it gets confused with wrist flicks. When I have a day at work where I am flipping through hundreds of pages of papers, I hit my 10k steps before lunch... from having sat at my desk. But you can go on the website and log that activity and it will remove the steps. Sometimes it counts steps while driving, but I have an app on my phone if I care to use it that will automtically log my driving as such on the Fitbit site. Also, sometimes pushing a grocery cart vibrates too much and puts it into sleep mode, but that's easy to undo. Overall, I will take the inconsistencies and hope they fix it in updates because it's so nice to forget I'm wearing it.

I had my Flex for a month before I got my Polar FT7. I thought, for whatever reason, that my Polar would track my HR from my wrist. But it tracks from my chest, so if I'd want to know my HR all day, I'd have to run the battery down to keep it on all day.

I love my Flex. The bracelet is cool: no one ever thinks anything of it. I'm even planning on wearing it to my cousin's wedding. It's black, so it goes with everything. I've found that if I'm sitting in a chair that rocks, it picks that up as steps. Or if I swing my arm as if I were walking--that's counted, too. Otherwise, my flex seems to be accurate. When wearing my HRM and my flex, they decided I burned the same amount of calories.

Something that people have mentioned before was accidentally losing their One or Zip Fitbit products. Or washing them. With Flex, it's always on your wrist. And for people that have smart phones, it's so easy to control the Flex from your phone. My family gets excited to go on walks with me now because they like to hear how many steps we've taken. I've never had my Flex miscount steps as I'm exercising. I jogged in place the other night and my arms were still. It counted almost up to 600 steps. Sooo. I don't know why it's inaccurate if your arms don't move!

The manual for the Flex encourages people to put it on their non-dominant wrist. So make sure that you do that! Otherwise, it's probably fine. Good luck with whatever you choose!

Just a note on the altimeter on the One - the reason Fitbit says they didn't include it in the Flex is because it's faulty and doesn't accurately judge your steps on an incline, so don't use that as a deciding factor.