I flew in the yellow (RAF training colours) Moth at Barton in about 2003 - before I started my PPL but after a few years gliding. The aircraft was in nice condition and the pilot let me fly most of the flight from 500' upwards - he said he expected that a glider pilot would feel at home at the controls, which turned out to be true - he gave me speeds and let me fly.
He also did a go-around to give me 2 looks at an approach (he flew them obviously). The whole flight was about 40 minutes but I gathered he'd given me quite a bit of extra time above and beyond the allotted.
A very good experience, thanks to my nice son for the birthday present.
Subsequent experience of both airfields would lead me to recommend Sywell, only because the surroundings are prettier than Barton (hope that doesn't offend too many...) but bear in mind it's quite tricky to book these flights - you have to phone book then ring on the day and flights are often cancelled due weather.