When comparing Fitbit Flex vs Fitbit Flex 2, the Slant community recommends Fitbit Flex 2 for most people. In the question“What are the best fitness trackers that pair with iPhones?” Fitbit Flex 2 is ranked 4th while Fitbit Flex is ranked 12th. The most important reason people chose Fitbit Flex 2 is:
The fitness tracker is completely waterproof and keeps track of your laps, pace, and total duration of your swim.
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 Great option for swimmers
The fitness tracker is completely waterproof and keeps track of your laps, pace, and total duration of your swim.
Pro Sleek design fits a variety of occasions
The Flex 2 is a thin band all the way around that has a modern and sleek design. Since it isn't super sporty in appearance, you can confidently wear it for a lot of different occasions without it looking out of place.
Pro The app and web platform do an excellent job of keeping you motivated
The Fitbit app lets you set goals and milestones which reward you with badges and trophies when reached.
You can also challenge friends and family, competing against them for top spot on the leader board.
Pro Replaceable wristband with lots of options available
The fitness tracker itself is a separate part from the wristband that can be taken out and put in a different band. Therefore, not only are there 10 wristband colors to choose from on the official store, a different color of band can be worn every day with the same tracker. There are even custom bands available from third-party vendors.
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 Underwhelming battery life
The fitness tracker's battery lasts about 3-4 days on average.
Con Can't be paired with external HR monitors
The Fitbit Flex 2 fitness tracker has no built in heart rate monitor and lacks the ability to pair with external HR monitors.
