A:AnswerYour wrist size. Unless you have a very small wrist, I would get the large. I purchased the small at first and it didn't leave much room. Also, there are a ton of beautiful replacement bands to go with it, sold separately.
A:AnswerThere is no technical difference between the two. Inside they are both the same. What you are paying a premium for on the gunmetal one is the gunmetal style, i.e. the gunmetal holding frame and watch band.