It depend on your resolution. Read this article and decide. Generally 560 Ti is better but only 1 tier higher than 6870...Most likely even on 1080p you won't notice a difference http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
Are you crazy!? The GTX 560Ti is leaps and bounds more performance than a 6870.....
GTX 560Ti is on the same performance as a 6950. An overclocked GTX 560Ti is on the same performance as a GTX 570.
You WILL notice a BIG difference in performance. The GTX 560Ti is a far superior card, ESPECIALLY when Over-clocked.