Hendon is indeed an old RAF airfield (you can see the open area and old hangars from the museum parking space), and a brilliant Museum.
That said, Duxford is absolutely awesome.
Not just a museum, - you can expect a Spitfire zooming over your head on a good sunday.
Look at the IWM website, you should find Duxford, then you can see the staged events. They have really some stuff.
I've seen 24 Spits in the air at the same time more or less, some mad aerobatics by them, and even WWI stuff, Se5's, Bristol fighters, Sopwith tripe, and a FOKKER DR I doing loops!!!!
For a rainy day, look at the IWM in south London, - not so much aircraft (although you can walk through some bomber fuselages and see Rudolf Hesse's 110 wreck as well as viewing aircraft hanging from the ceiling), but it has a lot of other material and is well worth the visit.
Best of luck and have fun!