In addition to where are you staying, 2974, is when?
That is critical because of the air shows at each museum - flying ones at Duxford, static at Hendon, less frequent with less flying at Cosford - happen on certain days which would either encourage or discourage you from visiting on those days.
In my opinion the flying at Duxford is incomparable. Most excellent.
Duxford is much bigger than any of the others, with a particular concentration in WWII and especially the USAAF and USAF involvement in it. Almost everything at Duxford is flightworthy or being worked on to be. Getting there can be a bit of a picnic for tourists but I think it is well worth it. It is an active airfield and even on non-flying days there is usually something moving. They do a decent air raid mockup, too.
Cosford has a lot of planes, some in better nick than other samples. Much less crowded, in Shropshire, with a train station adjacent. More civilian planes than at Duxford which is almost all military.
Hendon (when we say Colindale that is the same place) is entirely static but what they do they do well. Entirely military.
If you are here during the Farnborough International Air Show, that is when you can see current and prototype aircraft from civilian to military both static and flying performance demonstration flights. It is primarily a vehicle for selling aircraft and missiles to governments around the world and airliners to airlines all around the world. It is where I saw a 747 performing acrobatics, some years ago.