Just a quick note regarding fitbit. I have one. It works great. Excellent product. But unfortunately Fitbit has decided to not integrate with HealthKit. iOS is fitbits largest market, so the decision is very strange. Health kit is a integral part of iOS. Therefore this is essentially Fitbit saying we don't support ios anymore.
My guess is they are up for an acquisition or looking for one. I've spoken to a few devs and founders in the health space and although many are annoyed at how healthkit was launched, the value of integration is completely obvious. This value is obvious for both developer and consumer, which again makes Fitbit's decision really weird.
By the way, if they don't have it already, every other platform - Google, Samsung, Windows etc will have their own centralised health data store which allow health apps to talk to each other. So is Fitbit not going to support any of these? So this isn’t an iOS problem this is a problem that is going to affect you no matter what platform you use. Your device is going to be crippled by this lack of support.
What about the Fitbit API? From a development perspective. The Fitbit API is ok - but if i'm building an ios app, i'm going to get step data from healthkit which will allow my app to support any device. I don't want to write code for each individual device. From a consumer perspective - what the hell is an API? From personal experience it works 50% of the time in the myfitnesspal app.
So looking to buy a fitbit and use ios? Don't. They no longer support ios. There are numerous other options that do.