Bungie has just announced that for a limited time, when you buy a last-gen (PS3/Xbox One) digital version of Destiny, you'll also get the next-gen (PS4/Xbox One) digital version for free. Yes, free; the only catch is that the free upgrade will only transfer within the same console family, meaning PS3 to PS4 and Xbox 360 to Xbox One.
According to Bungie, if you buy/redeem the Destiny Digital Base Game, Guardian Edition, or Expansion Pass for PS3 or Xbox One on or before January 15, 2015, you can download the PS4 or Xbox One version for free. What's more, you'll still be able to play the last-gen version of the game even if you download the next-gen (err, current-gen). So you're essentially owning the two copies of the game, one for each of the generations.
And as we've already learned, your Guardians, progress, stats, and gear will carry over from last-gen to current-gen, again, as long as it's within the same family.
For more details, including how to actually redeem this offer, check out the Destiny FAQ page.