Best option is a private car. Other option is the day bus which will be much cheaper.
Tam Hahn or Sinhtourist is the only recommendation I could make for Coaches. Nice trip takes about 4 1/2 hours with 1 stop. Small Coach usually.
Private car is around $90 and you could refer to saigon2muine website for details.
The best option is doing that by private transfer!
Best travel in Vietnam invariably is by plane or hired car.
It's a question of convenience and comfort vs cost.
If there are more of you a car would make sense.
Otherwise the day coach is a perfectly good option, even if it's not the "best".