The Polar RS400 is a lot more advanced than the RS300x. It&0s best selling points are that it is able to record your training data every 1 second of every session, it comes with Polar Pro Trainer 5 software, Polar Training Load, Polar Smart Calories, Own Zone & a Fitness Test. This watch is the choice of serious runners, as well as other sports (such as footballers) because of it&0s data recording abilities. In The Box Polar RS400 Polar RS400 training computer Polar H1 heart rate sensor Polar Pro Trainer 5 software CD RS400 Getting Started Guide Free Irda USB