Virginblue,
Traffic and timings on buses usually very reliable, especially leaving from the bus station as most exit directly out onto the motorway and and quickly on their way south. If there was an accident and a tail back, certainly no slack in the schedule for that. I'd take the earlier one. There are buses on the hour and half past the hour. Travel time quite consistently 2 hrs.
Flying at 955 (guessing FR To STN or BHX, or BE to CWL),
Check in online so go through passport control at 830, to be on the safe said, though passport control very quiet at that time.
I'd actually take the 6 bus! Or 530.. To be early.. If you need more info, pm me...