5700kv is similar to a 9 turn brushed motor.
7700kv is similar to a 4 turn brushed motor.

7700kv is faster, but uses more energy=less runtime

5700kv has an even balance of torque and speed.
7700kv is more for speed.

What are you putting the brushless kit in, and what do you intend on doing with it?

RC Car magazine reviewed them both a few months ago, very informative.

My HPI Sprint 2 touring car has the 5700KV in it, and its plenty powerful enough.