Fitbit has set the standard in wearable fitness trackers. They have the most name recognition and market share of all devices in the segment. They have a variety of products available with different features to suit the users needs. Alta is also a relatively unknown device with little brand recognition. I have heard very little about it to date.
The Alta and the Fitbit are both on the same level. They both serve the same purposes when it comes to keeping track of one's health. The small differences between the two may make or break a product to some, but Alta is not better than the Fitbit and vice versa.
News articles about Alta such as TheVerge.com describe Alta as if it were a fashionable form of FitBit. Its functionality appears to be about the same as FitBit, and it is attached to a leather band. Workout equipment doesn't necessarily need to be high fashion, however. People who work out don't dress up in their finest clothes; they dress up in comfortable workout gear.
I think it is with great certainty that Fitbit is the better choice in the domain of sports/mobile accessories. They have been around a long time and it was them who invented this type of accessories. They are way in advance technologically and will remain in the near future or more.