I was using a Lumia 800 but just bought a 520 so that I could play around with WP8 and not drop a lot of money on a device since I am still under contract. The 520 is definately not as nice as the 800 in almost every way. However, the 800 does have 7.8 on it and many apps and features are not there. It also runs a just a little slower since it is a 1.4 ghz single core processor vs a dual core 1ghz. The 800 is heavier, but feels like you can play baseball with it, the 520 feels cheaper, because it is. The 520 lacks gorilla glass, compass, camera flash and capacitive buttons do not light up + has an inferior camera (but is a lot better than expected).
If I was in your position, I would go with the 520 for now because its a WP8, but know that you will want to upgrade it to one of the higher end devices in the future.