When comparing Fitbit Flex vs Polar M400, the Slant community recommends Polar M400 for most people. In the question“What are the best fitness trackers that pair with iPhones?” Polar M400 is ranked 2nd while Fitbit Flex is ranked 12th. The most important reason people chose Polar M400 is:
The "Sport Profile" feature allows you to customize what stats you want to see for each type of exercise. For example, you may want the GPS on for running but not for a circuit workout. These preset profiles save you from having to set up the device each time you exercise.
Specs
Ranked in these QuestionsQuestion Ranking
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 Gives you a lot of options for customizing different types of workouts
The "Sport Profile" feature allows you to customize what stats you want to see for each type of exercise. For example, you may want the GPS on for running but not for a circuit workout. These preset profiles save you from having to set up the device each time you exercise.
Pro Gives you custom activity goals suitable for your level
The daily activity goal is a good way to find out how active you really are in your everyday life. Here you can choose your typical activity level from three options and see how active you need to be to reach your daily activity goal.
The time you need to complete your daily activity goal depends on the level you have chosen and the intensity of your activities. Age and gender also affect the intensity you need to reach your daily activity goal.
Pro Detailed reports
The fitness tracker offers excellent reporting including distance, speed, cadence, heart rate (if paired with monitor), ascent, decent, calories, recovery status, and more.
Pro Ability to add training notes
The app allows you to add additional notes to your workouts for better tracking.
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 Sleep tracking lacks detail and accuracy
If you are awake for an extended period of time in the middle of the night and then fall back asleep, the fitness tracker may not see the additional hours as part of your sleep.
Con Unattractive and bulky design
The tracker is quite wide and hard to miss, it's not something that can blend in with what you're wearing outside of sport/exercise type settings.
Con Short battery life when using the GPS
With the GPS turned on, battery life goes from around 24 days to 9 hours.
This is likely fine for people who will always have opportunities to charge it after their run. However for anyone hoping to use the GPS for longer activities, such as hiking, that may not be long enough to be useful.
