Digital Foundry has posted up a Destiny comparison, showing how the PS3 version compares to the PS4 version.
Summary: The PS3 version is almost exactly the same, except blurrier.
While the levels themselves remain the same in overall shape and size, foliage and rocks are less dense and occasionally missing, while the lighting also less impressive. Perhaps the biggest difference is that while the PS4 version runs at a native 1080p, Destiny on Sony's PS3 runs at sub 720p.
Interestingly, when it comes to gameplay, the 8 year old machine holds its own, with the amount of enemies onscreen remaining the same.
So it sounds like if you are opting for the PS3 version then you won't be too disappointed.