Don't know about the instruction at either place, but have flown to and from both fields quite a lot, so offer the following thoughts for what it's worth:-
Wycombe has full ATC with very slick, user-friendly controllers. Learning there will familiarise you with operations at towered fields and help you pick up good RT skills early on. Also, the hard runway may be usuable in conditions when grass strips are too wet and muddy. The circuit pattern is a bit daft cos of noise abatement requirements, but no dafter than at many places.
Waltham has a lot going for it as a traditional grass 3 runway field, so you can usually get a runway more or less into wind, and operating on grass teaches you useful airmanship stuff (short/soft field techniques, what happens when grass is wet, looking after the nosewheel and prop when on rwy and taxiing etc.) Waltham also has more of the ambience of an old-style flying club. There are lots of interesting aeroplanes, often going upside down, and a jolly clubhouse. Wycombe is very modern and professional, but perhaps lacks something in atmosphere.
Wherever you go, have fun.