The FitBit One appears to just be an iteration of the previous FitBit trackers. It adds a wake-up-via-vibration alarm clock and it syncs with Bluetooth 4.0 instead of syncing via a proprietary dock. Nice improvements, for sure, but the One appears to no longer come with a dock to facilitate easy charging.
The FitBit Zip is a new product. It's different from the One in several ways, but the two that I think are important are that it uses a non-rechargeable battery that lasts for 4 months, and oddly enough, that it is larger. As a side bonus, it's also much less expensive at only $60. I imagine that the reduced price compared to the $100 FitBit One will make it attractive to many more people.
Why do I think the FitBit Zip is a better option (at least for me)?
First, I think the larger FitBit Zip is actually a bonus for careless people like me. As a gadget nerd that is interested in weight loss, I have owned multiple FitBit trackers in the past but have recently stopped carrying them. Yes, I said 'multiple.' That is because I lost one and accidentally sent another through the washing machine. Partly this is because I am careless, but it's partly also because the traditional FitBit is tiny and lightweight. It's easy to forget it is in your pocket, and it's easy to not notice that it fell off your belt. I hope that the new form factor of the Zip will actually remedy this for me. It looks like the belt clip for the Zip is actually a little neoprene case that fits around the actual ovoid Zip tracker. It would be neat if FitBit released additional types of holders. Perhaps a keychain holder? A wristband holder?
The second reason I like the Zip is the fact that the Zip has a battery that lasts 4 months even if that means I have to keep a small supply of watch batteries at home. With the original FitBit trackers, I'd often forget to put it in my pocket the mornings that it was sitting on the charger (about once a week). With the Zip, I'll be able to just leave it with my keys and wallet each night and not have to worry about charging the battery once a week.
The Bluetooth 4.0 connectivity is a nice bonus for both new units. Previously, the FitBit tracker would only upload its data to my FitBit dashboard at the end of the day when I came within range of its dock at home. Now, it will update via my iPhone anytime I use the FitBit app. This is particularly helpful for people like me who are using the FitBit meal tracking features because it means that the app's calculation for how many more calories I can consume will be accurate based on an up to the minute understanding of my activity level so far for the day.
All in all, these are enough improvements for me to rationalize trying a FitBit tracker one more time.