Buy for Better Workout Tracking
The Apple Watch 3 includes new sensors that will upgrade your workout tracking. This isn't going to be enough of an upgrade for casual users, but there is now an Altimeter in the Apple Watch series 3 that will track how high you ran on a workout.
This will also help track stairs climbed when you have just the Apple Watch and you don't have the iPhone on you. If you do a lot of workouts with elevation changes and you want the most data and accurate tracking of your workouts this is a smart upgrade.
You also get the benefits of the watchOS 4 update that includes smarter workout options, but you can also get that on your older Apple Watch so consider that if you are an upgrader.
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