We have had several 520s, and recently bought a 532. Here are the differences I have noticed:
Build Quality: The 532 feels cheaper than the 520. The 532's cover seems to provide more protection from drops, but it is noticeably thicker. The screen viewing angles on the 532 are bad - color is markedly poor from above and below.
Performance: the 532 is noticeably faster. It also can run all apps and games as it has 1GB. The 520 stutters during games, and app loads are slightly slower than the 532. Boot time is slightly faster on the 532. Communication (phone, text) is faster and clearer on the 532. I do not agree with the other poster about the 520 being "significantly slower" than the current generation, but the 532 is overall faster.
Functionality: The lack of a dedicated camera button on the 532 is disappointing; however, if you've never had a dedicated camera button, this may be a non-issue. Having a glance screen on the 532 is great. Both take expanded storage. Both speakers sound about the same. The 532 has a front-facing camera, if selfies are important to you, but the 520 has an auto-focusing rear camera. Still I think the rear camera on the 532 takes better shots.