When comparing Fitbit Flex vs Fitbit Zip, the Slant community recommends Fitbit Zip for most people. In the question“What are the best fitness trackers that pair with iPhones?” Fitbit Zip is ranked 8th while Fitbit Flex is ranked 12th. The most important reason people chose Fitbit Zip is:
The Zip keeps you motivated through friendly competition. Along with giving you a rank among your friends, you can also cheer them on or taunt them.
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 Social component to help keep you motivated
The Zip keeps you motivated through friendly competition. Along with giving you a rank among your friends, you can also cheer them on or taunt them.
Pro Easy to hide under your clothes
The Fitbit Zip fitness tracker is very small, so you can easily wear it somewhere that it will be hidden (belt, pocket, bra strap, etc.). Unlike most trackers which have wrist bands, you don't have to worry about trying to make it blend in with what you're wearing.
Pro Fitbit app lets you track a variety of health-related stats
The app doesn't only display the stats from your device, it also lets you track a variety of different health-related stats by entering them manually. You can log your food for the day, how you slept, activity, weight, and more.
Pro Automatic synching that's easy to setup
The Fitbit Zip fitness tracker wirelessly syncs with both PC and smartphones via the Fitbit app. After getting it set up you don't even think about syncing, it's just automatic.
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 Lacking many of the features that fitness trackers often offer
The Fitbit Zip only tracks distance, steps, and calories burned. It's basically a pedometer with a smart tracking platform you can use for manually tracking the rest of your day. It can't tell you stairs climbed or heart rate, and there is no alarm, sleep tracking, or GPS.
Con Issues with the battery not working
The contacts in the device are very sensitive. If you don't place the battery in just right, it will only work for a short period of time before the battery needs to be readjusted or replaced.
Even when following instructions provided by the company, some people report that still can't get it working properly. The battery may end up lasting days instead of the promised 4-6 months.
You can see how to properly change the battery here.
Con Easy to lose
The tiny size of the Fitbit Zip makes it very easy to lose track of.
Rather than a wrist watch like many other trackers, it only has a slide on clip. The clip is fairly secure for normal activity, but you may knock the tracker off without realizing it.
Con Lacks a rechargeable battery
The Fitbit Zip fitness tracker runs on a 3-volt lithium-ion battery that lasts between 4 and 6 months. It requires getting a new battery every time it runs out of charge.
