The Nokia Lumia 720 looks sleeker, has a longer lasting battery, NFC and an amazing display along with an amazing camera. However the affordability of the Nokia Lumia 520 which packs a decent set of specifications makes it a worthy device capable of taking on (and besting) most of the affordable Android handsets in the market. If price is not a factor the Lumia 720 will be a clear winner, however you wont get a better performer for the cost of your Lumia 520.
You can read a detailed comparison of the Nokia Lumia 520 and the Nokia Lumia 720 on PhoneArena.