I have the 305. The main difference though is the heart rate monitor. I bought the 305 because it really didn't cost much more than the 205. I do not use the heart rate monitor often, but it is nice to have, if the cost is not too drastic. With the 405 out, you can find the 305 at pretty good prices.
The garmin is the best running purchase I have made. It is great when traveling. You can map a route out online, which is what I used to do, but it is nice to just change directions once in a while. It has also really come in handy in races too. I have the time and distance right there on my wrist. I do not have to wait for a mile marker to see where I am and how I am doing. I have been in some races where there was not a marker at every mile. It gives a pace too, but I do not think that is 100% accurate. I think the pace is more reliable if you are running fairly straight at a fairly constant speed. I still look at it in races sometimes to get a sense of how I am doing, but I would not rely on it too much.