The Nokia Lumia 720 may have a clear advantage over the Nokia Lumia 620 with its larger 4.3 inch display, battery life, call quality and slim unibody design. But its biggest advantage comes from its camera low light enhancements and improvements. However the Lumia 620 boasts better affordability and its compact design in better suited to single handed use. The Windows Phone 8 experience between the two handsets is almost identical, so the final choice may come down to personal preference.
You can read a detailed comparison between the Nokia Lumia 720 and the Nokia Lumia 620 on PhoneArena.