Series 6,7,8, and 9 in theory all the same heart sensor, not sure of the processor makes much difference in the heart monitoring functions, but if it does go for the 9, otherwise, get the cheapest you can find out of the series 6-8. The ultra’s would be as good as well. One thing to keep in mind the watch band matters, finding the right fit will help keep the heart rate monitor accurate, so find a band that fits well.

No Mac support makes this a pointless app for me. I don’t like typing on my iPhone much, especially if I’m trying to type a journal entry. I was holding off getting day one for this so I can use this to make entries on my Mac, but still have it on my phone, I’ve been using the notes app, but it’s limited. They should wait until the app is ready for iPad, Mac and iOS, but it is what it is.