Tofu, I have the 405 and really like it but got to check out the new 310 at the expo at the Memphis marathon last weekend and would buy it over the 405 if I was buying new. The features I liked about it is that you can use it swimming, biking and running and I was told that it would switch between activities automatically and calculate your transition time. The 405 is only for running and biking and you have to go into the menu and change the mode manually. The other thing that is nice about the 310 is that you can buy a different watch strap for it so that you can twist the face of the watch off and then snap it onto a bike mount. The gentleman also said that the calories burned computation is more accurate on the 310 and the battery life was a bit longer. The only thing I didn't like about the 310 is that is was larger than the 405..................(wish they would come out with a lady's model that would fit on our wrists). The price difference was very slight and i actually think the 310 was cheaper. Of course, it's all personal preference and would depend on what your training needs were.