No dilemma for me:
Fairford: enormous queue to get in, packed to the brim crowd line, an overly-long display, constant battering of loud jets, then another enormous queue to get out again...
Duxford: no real queue, lots of people but not to the point of overcrowding, a three hour flying display (just right), the wonderful sound of Merlins, Griffons, R2800s,etc - and this year two Mercurys attached to two different Gladiators (fingers crossed!), and no real queue to get out again.
I haven't been to Fairford for about 15 years and I don't think I ever will again. It's just too big. Give me Duxford every time.
Whichever you choose, enjoy it!
Treadders