Note: 5770=6770
Shows that the minimum framerate is a little bit better overall in the 6770/5770, and the average fps is a little bit better on the 550 ti. So I think they perform the same overall.
The main difference is Power consumption, because performance is the same. So look at this table:
You can see 6770/5770 consumes far less power than the 550 ti
In terms of price, I looked up newegg and:
6770: ~$120
550 ti: ~$130
So the conclusion is, I vote for 6770, mainly because of the power consumption. Price and performance should be the same.