Herndon and Northern Fairfax County still have some of the lowest crime rates in the DC metro area, where Loudoun County has seen a recent uptick in crime, especially violent crime. Both places are still light years ahead of lower Fairfax County, PG County, SE DC, etc. Schools are better outside of the town limits, Oak Hill, Great Falls, Centerville, Chantilly, etc. have great school systems.
Depending on where you come from, expect to pay significantly more in what you pay in rent now. Auto insurance is cheaper then MD and DC. If the prospective work location is South (more towards Oak Hill, Chantilly, Centerville, or near route 50, then I would consider south of Herndon and along 50 out towards South Riding area. I don't recommend the commute from Leesburg over to 50 area, during rush hour, it can be longer than it appears. Especially if your renting, rent near where you are going to work.