It's not a simple answer.
At 1920x1200 resolution the GTX560Ti 448 is faster than the 7850 in most games (including BF3).
At 2560x1600 resolution the 7850 is either faster than the 560Ti 448 or closes the gap. All settings at Ultra.
The 7850 will use almost 100 watts less under load than the GTX560Ti 448.
The 7850 sells for $260 average, while the 560Ti 448 sells for $315 average.
SO, you want pure performance, go for the GTX560Ti 448.
However, the 7850 will give you 95% of the performance for 80% of the price, while consuming less power.
For that reason, my vote is for the 7850, as it seems to be better bang for the buck. If you can find the GTX560Ti 448 for less than $280 though, then that would be the better deal.