On the speed/cadence sensor issue - I know, at least on my ForeRunner 310XT, that GPS speed would be quite inaccurate on any hill climb, because the unit was only calculating speed based on horizontal movement, which of course was less the more you were climbing. There's a good chance the 500 does a better job than that, and certainly after the fact it can, but traditionally that's one reason you want that sensor.

Or if you bike where there are a lot of trees.