Bungie and/or Activision must be feeling quite generous right now because they just announced a pretty great limited time incentive for those who decide to purchase the PS3 or Xbox 360 digital versions of the Destiny.
Basically, if you buy Destiny for the PS3 or Xbox 360 from the PS Store or Xbox Live, you will be entitled to download the PS4 or Xbox One digital version (under the same account and platform family), for free. This appears to work with the Guardian Edition, the standard edition and the expansion pass and everything, though purchasing the next gen version does not entitle users to the last gen version.
The older generation versions will still be playable and no matter which platform you play on (in the same family), the game will retain guardians, stats and gear.
Check out the FAQ on the official Destiny site, and below.