While the final release of Destiny might differ a bit from what everyone experiences and sees during the beta, we've already gotten a sense of how the different versions of the game will visually compare.
Obviously, something on a last-gen console won't look as hot as a current-gen console release—as you can see in this video by DigitalFoundry. Still, the game is gorgeous on both platforms. It's also worth remembering that you can transfer your saves from last-gen consoles to current-gen consoles, should you buy the game on the PS3 and then later decide that you'd like the prettier version instead.
Destiny Beta: PS3 vs PS4 Comparison [DigitalFoundry]