I just wrote this to someone,so in case it’s useful:
I’ve had/tried several Fitbit models. Here are some things to consider:
1. Is it important to you that you have something that functions as a watch to tell time (so you don’t need to wear a watch plus a fitbit)? If so, you want the Charge, Charge HR or Surge. The Flex, One, and Zip do not have a digital display (The Flex has a row of lights to show you percent of step goal reached).
Also you may want a digital display if you do not have an iPhone or Android phone with Bluetooth to sync the fitbit, since you will then only sync the fitbit to your computer so you can’t see your step count while out and about.
2. Do you want to wear it on your wrist? The Flex, Charge, Charge HR and Surge are wrist-worn. The Flex can also be removed from its default wrist band and put into “prettier” bracelets (you can find a bunch on Etsy), but the others cannot.
3. Do you want to clip it to your belt? The Zip and One are non-wrist strap.
4. Do you want to wear it to bed to monitor your sleep (times restless/awake/asleep)? All EXCEPT the Zip can do that.
5. Do you want to monitor your heart rate? Only the Charge HR and the Surge do that.
6. Do you have a small wrist/hand? The Surge and maybe the Charge HR may be too big if so.
Also look at the website for the size of the wristbands – the Flex comes with both Small and Large bands, but the Charge, HR and Surge come EITHER Small or Large. Also, if ordered directly from Fitbit you can return or exchange for 30 days if it isn’t right.