IMHO both are accurate if interpreted correctly. This is based on my experience with 4-12 hour interstate trips.

Yahoo seems to give you BIS travel times as if you never stopped, even on all-day-long trips, and does seem to account for real-life (not posted) speeds.

Mapquest seems to consider real-life pit stop delays. Neither seems to do a good job of anticipating delays due to heavy traffic or construction. In other words:

Actual trip time = Mapquest time + traffic delays = Yahoo time + delays + how long it takes to stop for gas, food, potty, etc.