I have both the 705 and 500. I use the 705 which I move between 3 road bikes and the 500 on my Tri Bike. For both of them, I really only use basic functions, that is Speed, Distance, HR, Cadence and Grade.
The KEY pro and con that I find between the two is readability. I wear glasses which I don't wear when riding so that compounds the problem with the 500. In very bright sunlight or rainy muddy conditions, the 500 is difficult to read. I had to reduce the number of fields on the 500 main screen to 3 in order to see anything. I do however like the mount for the 500, it seems much more secure.