I agree with BP about the potential aircraft renting restrictions post-PPL if you have learnt solely on a diesel.
Regarding the club choices available, why not visit both airfields and ask for a guided tour of the facilities including the actual aircraft, engineering - even ATC (they're actually very friendly at Wycombe....!). Ideally take a trial flying lesson at each and see how you get on - the time will count towards your PPL regardless of where you eventually decide to learn.
ATC and hard runways does not mean yuo have commercial traffic to contend with - no more at Wycombe Air Park than at White Waltham. The chances of seeing a B737 at either is retricted to looking up to see the traffic operating out of Heathrow 1000s of feet above.
If cost is important, remember the fleet at Wycombe includes Cessna 152s which work out cheaper.
Also, check how the flying rates are worked out. It may be on engine time, brakes on/brakes off or some other method - it can make quite a difference. Another cost to take into account is membership fees.