The GTX 560Ti is slightly better performance-wise. Take a look at the below benchmarking site and try to base your purchase on the results you see there. The GTX 560Ti comes in at rank #9 while the Radeon HD 6870 is at #13.
PassMark Software - Video Card Benchmarks - High End Video Cards
You should also read the following thread. It has some useful tips for people looking to upgrade their graphics card.
Planning to upgrade your Graphics Card? Read First...
Lastly, I wouldn't go with a Novatech card for two reasons. It's not as reputable a brand as ASUS, EVGA, MSI etc. Also, it only has a single fan. If you're willing to spare the £20, I would go for the
ASUS GeForce GTX 560Ti DCII. The problem with this card however it is overly large and you will find it very difficult to SLI if you want to.
If you're willing to spend an extra £23 you can get a
MSI GeForce GTX 560 Ti Twin Frozr II/OC. This is an excellent card that is factory overclocked higher than both the Novatech and the ASUS card. It also has Twin Frozr II fans which provide great cooling.