Well, deciding which platform to purchase Destiny on just got much simpler. Bungie has revealed that those who digitally purchase Destiny on PS3 or Xbox 360 (i.e. via PSN or Xbox Live) can redeem a free upgrade to the downloadable version of Destiny on PS4 or Xbox One. The program applies to all Destiny "content," including either the standard or "Guardian" editions of the game itself, as well as the expansion pass. Those who redeem their free upgrade will be able continue playing their previous-gen version as well. In other words, if you upgrade your PS3 purchase to PS4, you can still keep playing your PS3 version if you wish.
Naturally, there are a few restrictions. First of all, the upgrade has to stay within a given console "family," which is to say you can only upgrade PS3 to PS4 and Xbox 360 to Xbox One. You can't, for example, upgrade from PS3 to Xbox One (what do you think this is, Diablo 3?). Save data is transferable between versions but, again, only within the same console family.
Finally, this offer does not apply to any physical, boxed version of Destiny. If you pick up a physical copy of the last-gen version of Destiny, you don't get to upgrade it for free. Just to make things a little more confusing, the expansion pass included with some physical editions of Destiny can be transferred to a PS4 or Xbox One, but you'll need the a PS4 or Xbox One copy of the full game in order to use it.