These two cards can be quite confusing. As can others I guess. the point is that with these two cards you are getting quite a lot of difference performance wise between the standard cards and the factory pre over clocked cards.
To me it would depend on price and which version of the card we were talking about.
I'm in the UK and here we have no contest really as the 7850 is priced considerably cheaper than the 560Ti is now which takes the 560Ti out of the equation on a price/performance basis.
So it really depends on your local pricing and which card we are talking about.
In general the 7850 would be the better option unless you can get one of the faster pre over clocked 560Ti cards for a similar price to a standard 7850.
Mactronix