When comparing Fitbit Flex vs Fitbit Surge, 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 Fitbit Surge is ranked 18th. 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 Continuous heart rate tracking eliminates risk of forgetting to activate the feature
The Fitbit Surge fitness tracker has automatic, continuous, wrist based heart rate tracking. Heart rate data is stored at one second intervals during exercise, and at five second intervals during non-exercise activities.
Pro LCD screen that can display multiple stats without the need of using the app
Similar fitness trackers depend on Bluetooth in order to relay info to a smartphone. Fitbit Surge can display information locally on the display which is far more convenient.

Pro Lets users log meals by voice
Users can tell Surge what was eaten, and a database will be used to match the entry with nutritional information.
Pro Does not need to be recharged frequently
The battery lasts 5-7 days which is one of the longer lasting options for rechargeable fitness trackers.

Pro Easy to operate compared to display-less options
While Surge easily syncs with iOS, Android, and Windows phones, it does not depend on them for functionality. The touch screen and physical buttons are a huge step up from display-less models.
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 Might cause skin irritation for some
A number of people are reporting the Surge causes skin irritation. According to the company it's a very small percentage of customers.
Con Screen prone to inadvertent touches
It's really easy to accidentally brush up against the fitness tracker's watch and end up on screens that the user wasn't intending to navigate to. This forces you to swipe back to the correct screen.
Con Fairly big investment
While it's one of the most advanced trackers that Fitbit offers, it comes with a hefty price tag.
