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.
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.