The 560 Ti is faster. That's all you "need" to know. On average, it is 9-12% faster.
The 560 has 336 Shader Units, while the 560 Ti has 384. They are largely identical besides that (same ROPs, nearly the same clock speed, same memory speed and bus width).
In some ways the 560 is closer to the 460 design though. It has the same Shader Units, ROPs, memory bus width, but a MUCH more aggressive clock rate and also faster memory. The 560 is, on average, 13-19% faster than the 460.
The regular 560 has one of its streaming multiprocessors disabled. That gives it 7 polymorph engines and 332 shaders. The 560 Ti is the "full" version, with 8 polymorph engines and 384 shaders. The extra polymorph engine should help as more games start to feature tesselation, and of course the extra cores are always a plus.
The twin fan versions are going to be more cool efficient thus overclocking will be more stable. You'll probably find twin fanned cards are already factory overclocked which means you'll have a slightly faster card. I myself am going for the GTX Direct 580 CUII card as it has two fans, and you always want your machine to be as cool as possible.