As it happens I know all three reasonably well.
All are good, all are a bit on the expensive side - presumably because of their relative closeness to London, all three have schools who look after their aeroplanes and employ good instructors.
But for me the best of a good bunch is White Waltham. It has the best choice of aircraft, and a social side which knocks the other two into a cocked hat - nice clubhouse, lots of syndicates, regular evening social events. Basically, everything is that bit better than the local competition.
PA28s and C172s are equivalently good aeroplanes and poor trainers that are very similar to fly. There's nothing to choose between them - it's the instructor that matters. The C152 is a bit better training aeroplane and available at Denham and Booker but not at WW. On the other hand Waltham has Super Cubs and T67 Fireflies available, which in my opinion are far superior as training aeroplanes than most other offerings, because they both force much higher standards of handling technique (but are also more expensive.)
PA38s are good training aircraft, but unpopular with schools nowadays because of longevity issues. But I don't think that any of them are using Tomahawks any more are they? Booker used to so I could be wrong.
G