I'm looking for some real world observations on the battery life of the two sizes for the Series 2. How much battery life are you getting out of them, especially when you use the fitness functions on a regular basis. The primary fitness function I would use is the GPS. I've been looking for a GPS watch for my running (usually about 3 times per week anywhere from 3 to 6 miles) that also has good smartwatch functions. This seems like the way to go, especially since I already use Nike+ Run Club app on my 6S, and I could get the Nike+ watch. As I've looked at posts about the different sizes, I feel like I would need to go with the 38mm because I have thin wrists for a guy (just over 6 inches, or 155mm), but I'm a little concerned about the battery life with a smaller one than the 42mm. In the couple days I've looking at the forums, I've noticed people putting how long they use their watch each day, but they very rarely say what size they have. I think that's a crucial piece of info for those of us who are undecided. Thanks.