With that said the 560Ti is about 6 percent faster than the 6870.
So it can handle the 560 Ti, but would it be any safer to go with the 6870? You mention at the end that the 560 Ti is 6% faster than the 6870, but is that negligible in exchange for a greater assurance of the PSU handling the GPU under load along with the $50 price disparity?
This is obviously a much more subjective question, but I'd appreciate input either way.
I'd personally go with the 560Ti, your PSU is more then good enough dude.
Guru 3D recommended a 500 PSU, not because you would be pulling that much from the system (at peak you will only be pulling about 319w), but because they think that is the safest wattage to go with. Remembered thats STRESSED out only pulling 319w, you are perfectly safe using a 500w PSU with a GTX 560Ti. Hell you can run SLI 460's off a 550w PSU of good quality.
So yeah. The price difference is totally up to you, however. Personally I'd get the 560Ti.