I too perfer Xanax, but then again I never tooked Valium so I can't compare.
What I can offer is my own experience. I tooked (1) 1/4 mg for a three hour flight to Florida. I tooked one before my trip to see how I reacted to the med. It worked great, calmed me right down. So on my 3 hour flight, I waited till I boarded the plane, got comfortable, waited before take off and popped that pill. I had maganizes along for the trip as well. Best flight I ever had.
When I got back, I booked a flight to Las Vegas - 6 hour flight. I did the same as my Florida trip, tooked ithe Xanax when I boarded the plane and was settled in my seat - tooked one, waited an hour and tooked another for the 6 hour flight. 6 hours went by so fast. I even tooked in a movie and nap.
Now, I'm working on taking a trip to Hawaii, that's a 23 hour flight for me.
I'm going to have a stop over in CA to break up the flight - but I haven't a clue what to do on my return trip
When I figure out what to do - I will plan my trip.
Enjoy your flight and trip.