I have 2 Series 4. 1 I wear to bed (Sport), and the gym in the morning. The other (Stainless Steel) one for the rest of the day. It works great for me and I do not lose any activity data.
On which app is best, I suggest giving them all a try. I have used Pillow, AutoSleep, SleepWatch, and Sleep ++. They each have their goods and bads. I use a combo of AutoSleep and Sleepwatch. AutoSleep has a good complication that i can quickly see to tell my sleep "deficit" for the week. SleepWatch will ask me 2 questions every day. In the morning it asks me how rested I feel, and in the afternoon, how fatigued I feel. It then builds trends using that information. it will also tell me when I have a better quality sleep due to exercise. If I had to pick just 1, probably SleepWatch, but I do like the complication of AutoSleep. Again, play around with the free version of them all. You can use them at the same time and compare.
