For the new URL format, cf the discussion about opaque keys and split mongo:

* Opaque keys discussion: https://groups.google.com/d/msg/edx-code/B6tRqQch9n0/vmgSg667oxMJ
* https://github.com/edx/edx-platform/wiki/Opaque-Keys
* https://github.com/edx/edx-platform/wiki/Split:-the-versioning,-structure-saving-DAO

So until the new key format lands, we should likely go with the second option, and then migrate to the new key format.