I just checked IAD-LHR-IAD flights for May 17-21. C fares are as follows.
Nonstop: $6252 CO and UA; $6352 BA; $6608 BMI; and $8564 Virgin. (Can flying Richard be worth this premium?)
One-stop: $5869 AC
An even curiouser variety for LHR-IAD-LHR the same days:
Nonstop: $7343 UA and CO; $7419 BMI; $7495 Virgin; $7653 BA
One-stop: Starts at $2784 on SAS, $3840 USAir; $3886 DL; $4517 KLM; $5427 AF. Immense premium for non-stop.
I could go on. The moral is: fares can vary immensely. Unless you are very sure that one flight is going to be superior to another, you might as well keep as much money as you think reasonable in your pocket.