My 305 works fine for marathons and I've used it for seven hour runs without problems. That's a lot of extra money for some loading automation and a 20 hour potential. If mine breaks I'm getting another 305.
Rich it depends on what your needs are. I own both the 305 and 310 and the 305 I use everyday as a workhorse. But when racing I always use my 310 because I run negative splits and I need the virtual partner mode to be not only on, but also I need to be able to change the pace on the fly without having to reset watch. The 310 also connects to satiliate much faster and is more accurate. The fact it is also waterproof is a huge plus. So on this basis the 310 is definitely worth it, although I do like my 305 for just everyday use
I have to say Rich for me the 310 is definitely worth it. The 310 connects to satilite much faster, it also allows one to change the pace of virtual partner on the fly as I run negaive splits. The 305 does not allow you to change pace of virtual partner without resetting watch so that is not good. The 310 is also more accurate. I do use my 305 as my everyday GPS when I do recovery runs and not so serious runs. I use my 310 for races.
Cazares, I own both the 305 and 310 and yes there is a big difference. The 310 connects much faster, is more accurate, and the virtual partner pace can be adjusted on the fly up or down without having to reset workout like the 305. I also like the ability to upload wirelessly to my computer where the 305 does not have that ability unless you don't mind connecting via usb