which school district is better. Loudoun County Public Schools or Fairfax County Public schools?

Fairfax, not even close. The best SAT school in Loudoun, would fall 9th on the Fairfax County list. The schools here are more established and building on what they have, whereas in Loudoun resources are going to continue to get poured into their growth and building of new schools. Loudoun is not bad, but doesn't come to close to Fairfax from a testing perspective which is really the only way I know schools are evaluated.

Forget comparing the two school systems in general. Compare the specific schools.