The only real drawback to Ashburn is that it's kind of far away from DC. However, if the new job is going to be around Reston, then that's not a problem at all.
Another great area (just my personal opinion, of course) is around Clifton, Virginia. Check out http://www.cliftonva.us and http://www.cliftonva.org It's a small, quaint town; within a few miles of the town in any direction you can find some really nice homes, good recreational facilities, lots of activities for busy kids, and so on. It's 10-15 miles from Reston.
Hope that helps.
For riding, I'm assuming that you mean horses. Western Loudoun County is horse country while Mclean and Great Falls in Fairfax County are known for large properties and horses.
You can check out the Loudoun County Guide post I did on my site/blog - http://realdiablog.typepad.com/loudounstats/2008/04/the-2008
You can also find out more about Loudoun County from a real estate point of view in the "Buyer Resources" category on my site/blog - http://realdiablog.typepad.com/loudounstats/buyer_resources/
If you'd like more information about Reston, Ashburn or any other area in Northern Virginia, just let me know.
Use this link to look at schools in Loudoun County http://cmsweb1.loudoun.k12.va.us/loudoun/pages/static_distri
If you have any other questions please don't hesitate to ask!