Are you looking for one hotel for two nights (a night prior to Warrenton and a night prior to Sterling), or just the one night between Warrenton and Sterling? Also, what is your budget on your expense account?
Assuming one night between Warrenton and Sterling, there are many very good hotels near Dulles Airport, which is just south of Sterling and about 40 minutes from Warrenton depending on traffic. If it fits your budget the Marriott Westfields south of Dulles is very nice. If it doesn't fit your budget there are probably two dozen other choices at more affordable price points that will work for you geographically.
Assuming two nights, I'd probably spend one night in Warrenton and one night closer to Sterling. I wouldn't stay in Warrenton and plan to drive to Sterling in the morning on a week day, because the AM rush traffic on that route can be soul crushing when heading North / East.