I'm using a 310XT now having bought it used off a chap at work, going cheap (I couldn't resist). I used to have the 305 (my wife inherited that).
I like the 310xt a lot. It's predominantly a Tri orientated watch. While i use it for cycling and running I've not swam with it.
To be honest, if you are happy with your 305 I wouldn't bother unless you can get a good price on it. While features like the vibrate alert are good and the calories algorithm is better (uses heart rate etc) it isn't that much better than the 305. I'm sure the accuracy and speed of the GPS lock was better on the 305 too. Not that the 310 is bad you understand just that 305 seemed a bit more reliable.
I weigh around 93kg and with every pair of running shoes I've had so for I've fealt them go at around 200+ mile mark. If I have continued much beyond 200 miles then I have started to get knee pain. Currently in Mizuno (WI8), but a similar story for previos NB and Brooks.
I think the bottom line is that it depends on your weight and running running style.
I have some of the 2 in 1 shorts and like them a lot. I also have some Nike Tech Tight shorts which I prefer to be honest. I still haven't found a regular pair of baggy shorts that don't cause chaffing round the crotch.
Also, tights all the way in cold weather. Wear what works for you not what looks good.