When comparing Fitbit Flex vs Fitbit Alta, the Slant community recommends Fitbit Alta for most people. In the question“What are the best fitness trackers that pair with iPhones?” Fitbit Alta is ranked 5th while Fitbit Flex is ranked 12th. The most important reason people chose Fitbit Alta is:
For its size, the Alta fitness tracker has a large display. The display will show steps, distance, calories burned, active minutes, and sleep time. You can switch between these by tapping the side of the display (there is no button). The screen is easy to read at a glance while on the move except in bright settings.
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 Informative and clear display that's easy to read
For its size, the Alta fitness tracker has a large display. The display will show steps, distance, calories burned, active minutes, and sleep time. You can switch between these by tapping the side of the display (there is no button). The screen is easy to read at a glance while on the move except in bright settings.
Pro Many different wrist band options available to suit your taste
The Alta itself comes in 4 colors: black, teal, purple and plum.
While expensive, you can swap out the band. Classic bands cost $30, leather bands cost $60, and stainless steel bands cost $100.
Pro Reminds you to move every hour
The Alta will remind you about your move alerts each hour. By default, it suggests 250 steps each hour and it will vibrate 10 minutes before each hour is over to give you enough time to accomplish the micro-step goal.
Pro Will fit almost any wrist size
The Alta is available in three different sizes (small, large, and extra large). Each size can be adjusted by a minimum of 1 inch.
This is a great tracker for people with wrists below or above average size who may have trouble finding a comfortable fit with other trackers.
Pro Lightweight and comfortable to wear
As a result of the Alta having a slim design, it is both lightweight and very comfortable to wear.
Pro Week long battery life
The fitness tracker's battery can last a full week before you need to recharge it - quite a bit longer than most activity trackers.
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 Fairly basic features which limits the information you'll receive from the tracker
The Alta fitness tracker is designed to be more of a passive tracker for the Average Joe as opposed to a complete activity tracker for enthusiasts. There is no GPS or heart rate monitor, so the stats you get are limited to steps, rough distance traveled, estimated calories burned, active minutes, and sleep tracking.
Con The device cannot be used without a smartphone or a computer
In order to activate the device for usage, it must be paired with a smartphone and/or a PC (through a USB dongle), and an account must be registered through Fitbit.
Con Difficult to see the display when outdoors
The display is quite dim which makes it very difficult to read when in bright settings.
Con Display is off by default
To manually turn on the display you must double tap the side of the tracker which can be difficult to do properly.
It can also turned on automatically by the appropriate wrist/arm movement for checking the time.
There is no way to keep the display on so it may take some fumbling every time you want to look at the display.
