When comparing Fitbit Flex vs Garmin Vivosmart, the Slant community recommends Fitbit Flex for most people. In the question“What are the best fitness trackers that pair with iPhones?” Fitbit Flex is ranked 12th while Garmin Vivosmart is ranked 16th. The most important reason people chose Fitbit Flex is:
The fitness tracker itself is a separate part from the wristband that can be taken out and put in a different band. There is 10 wristband colors to choose from in the official store, and custom bands available from third-party vendors.
Specs
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Pros

Pro Replaceable wristband with lots of style options available
The fitness tracker itself is a separate part from the wristband that can be taken out and put in a different band. There is 10 wristband colors to choose from in the official store, and custom bands available from third-party vendors.

Pro Comprehensive reports of your daily activities
The fitness tracker offers stats such as distance traveled, steps taken, calories burned, weight changes, calories consumed from foods and drinks, restless and awake states of sleep, etc.
Pro Integrates with over 40 apps so you can get the info that's most important to you
The fitness tracker can be integrated with a variety of apps that together can offer more in-depth and complete overview of your health and fitness patterns.
Pro Visually displays your daily progress using LED lights
There are 5 LED lights that, after double-tapping the wristband, display progress towards a goal. Each light indicates 20% progress towards the goal and the tracker allows choosing between steps, calories or distance to display this way.

Pro Tracks weight goals
Fitbit Flex lets users create a meal plan and log their progress from the dashboard menu.
Pro 5-7 day battery
Fitbit Flex will last up to a week on a single charge.
Pro Real-time Bluetooth syncing
Flex constantly shares data over Bluetooth, meaning that the device doesn't need to be taken off in order for the data to sync to an iOS or Android device.
Pro Social layer can increase motivation
Information regarding goals and progress can be shared with friends, and users can even challenge each other with goals.

Pro Quick food and drink logging
Because of Fitbit's database of foods' nutritional information, food and drink logging is easy.
Pro Comfortable
The lightweight rubber band is very comfortable to wear.

Pro Smartwatch-like functionality
Vivosmart is capable of showing smartphone notifications and control music playback.
Pro Logs exercise
Pro Week's battery life
Pro Some customization options
Pro Displays time
Pro Good step counter
Cons

Con Oversensitive pedometer overestimates your step count
The fitness tracker's step counter often counts movement unrelated to walking or running as steps. It ends up reporting more movement than has actually happened.
Con Hard to get the tapping right to active LED display
The double tap to activate the LED display lights must be the appropriate pressure and timing in order for it to work correctly. This can be frustrating to some users.
Con Companion app is difficult to use
The way the information is presented is confusing and incoherent and there's no logical way to find necessary information.
Con No activity autodetection
Vivosmart does not automatically detect when you start to walk or run, you have to start and stop activity manually.
Con Reps counting feature does not work for some exercises
Reps counting feature is not reliable, sometimes it misses few reps and with some exercises it can count 1 rap, even if you did 10 reps for example. And it can't detect when you start the exercise, so you have to start and stop the counter manually. In practice I frequently forget to start and to stop the counter so after some time I simply sopped using this feature.
Con Difficult to check current time because display most of the time is turned off
In order to turn on the display, for example to see current time, you need to raise you hand and after some delay display some times turns on, sometimes not.
Other option is to double tap the display, which requires both hands.
If you frequently checking current time, it is really annoying.
Con App requires you to provide personal data in order to work
You have to register in the app providing information about your age, gender and weight.
Con App does not work without internet connectivity
It is not possible for example to enter your weight if you are not connected to the internet.
Con No GPS data is collected
