The main difference between the two is the 7.7fx has a roadrace oriented drivetrain being "105" and the 7.5fx has a mountainbike/touringbike drivetrain being Deore / Deore LX.
The funny bit is that the 7.5fx, with it's touring setup, has a larger gear than the 7.7fx
7.7fx has 50/12=4.1666 and this is smaller than the 7.5fx which has 48/11=4.3636.
$800 more sure seems like a waste here to me.
If you feel the need to go really fast and have average speeds above 20mph ... get the 7.7fx and ask your LBS to fit it with a 11-25 cassette or similar ... if however you are a normal biker the 7.5fx will be more than sufficient.