The 500 is an older model and I'm not sure, but I don't think they are making it anymore. The 520 does offer live strava segments if you are a premium strava member, if you are into that. the 500 does not. The screen on the 520 is quite a bit easier to read than the 500 and it is bigger. If my 500 ever died (and I'm frankly surprised it hasn't yet), I'd be buying the 520. Same general size, similar button layout, but just newer and more features.
This is the reason to go with a 500. I know numerous people that have issues with the newer ones, yet my 500's keep going strong. I bought another during black Friday as a spare. No gadgets, nothing fancy, other than the fact that it just works.