When comparing Fitbit Flex vs Garmin Forerunner 235, the Slant community recommends Garmin Forerunner 235 for most people. In the question“What are the best fitness trackers that pair with iPhones?” Garmin Forerunner 235 is ranked 7th while Fitbit Flex is ranked 12th. The most important reason people chose Garmin Forerunner 235 is:
It's quite lightweight at 1.6oz and has a large bright display for easy reading while on the move in all types of lighting conditions. It's water resistant up to 50 meters so you're safe to wear it running in all types of weather.
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 Ideal tracker for runners
It's quite lightweight at 1.6oz and has a large bright display for easy reading while on the move in all types of lighting conditions. It's water resistant up to 50 meters so you're safe to wear it running in all types of weather.
Pro Quickly picks up a GPS signal
It picks up the GPS signal much faster than most other GPS fitness trackers available, so there is little waiting around for it to connect before you start your run.
Pro Customize the display screen to display the data most important to you
The display screens are fully customizable so you can control which stats you want to see.
Pro Extremely lightweight and comfortable
Despite the amount of hardware this tracker contains and it's large size, it's very lightweight and comfortable to wear.
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 Has trouble tracking heart rate during intervals
During interval training when there is a frequent change of exertion, the Forerunner 235 has trouble continuously providing accurate readings. It's better suited for running with a sustained pace.
Con Needs to be recharged often
Garmin states the watch has a battery life of 11 hours training and 9 days of use as a watch, which is a fairly short charge compared to other fitness trackers.
However many people have found the actual charge to be much less, often needing to be recharged every couple of days.
