Well to answer that, I'd like to re-direct your attention to all the features coming with the Black update. Regardless to say all WP devices will receive GDR3 update.
- Glance Screen Notifs
- Glance Screen Night Mode Colours
- Glance Screen Pedometer, Custom text and Cal info
- RAW Camera support
- New Camera Algorithms
- Bluetooth 4.0 LE
So when it comes to the Lumia 720, it will receive all the updates coming with Black except for RAW camera support(Don't ask me why) and Bluetooth 4.0 LE (Because Lumia 720 got it with Amber, Before the high end Lumias).
As for Lumia 520 goes, it doesn't support Glance screen, so one doesn't have to be a rocket scientist to realize that the Lumia 520 wont support glance screen related updates (Or unless Nokia somehow enables it on the Lumia 520).
As for DTTW(Double Tap To Wake), Nokia decided not to ship the lower end Lumia because of battery issues. And please don't throw the gauntlet at me that the much lower spec'd Asha 501 has DTTW.
But if you do, please note that the Asha is already a lower spec'd phone(and smaller screen). So enabling the DTTW on it didn't affect battery as much as it did on the lower end Lumias.
And again lets remind ourselves a very crucial thing, The Lumia 720 is a mid-range device. A best of both worlds. So expecting it to support all the fancy stuff of the flag-ship device will defeat its purpose. I'm sorry if it upsets anyone, but that is the case when it comes to phones.