If you have not yet made up your mind, all these areas are close to airport and near amenities! But if I understood you correctly you are asking where would it be wise to live considering your given factors?
One important factor, I suggest you must consider where is work, and what hours will you or family be travelling to work. Because getting out to go to 28 S from Ashburn in the rush hr. may not be what you anticipated!!
I live in 20165 for last 9 yrs and have been very happy with the schools Elementary and Middle..South Riding you may find the newer house for the price but again getting to work is important factor,. If you dont care about that then I guess all these areas are good. Only difference you may see is Newer house for your money in a newer area vs. a little older in communities like cascades or Ashburn Village Blvd. areas.
Have you considered Ashbrook? that is new right on 7 and across from shopping, and approach is good on wither side?
Hope you find the best home for your family.
The best thing to do is to check out the areas on your own at different times of the week. Try your commute during rush hour, and also spend some time there on the weekends to get a feel for the communities. There are some really good deals in Ashburn and Broadlands. Lansdowne also has some good deals, too. The homes there are fairly new. I have family that lives there, and they love it!
My website http://(www.TinyVarner.com) has a direct link to the public face of the local MLS. This is the best place to look for a home...if it's for sale, you should find it there! Let me know if you have any other questions or if there's anything else I can do. I'm happy to help however I'm able.
Here is a website that will help you compare schools. http://www.greatschools.net