Well, I looked at both Rabbies tours and here is a brief itinerary of both.
Canterbury: Canterbury Cathedral, Dover Castle, White Cliffs of Dover and Kent.
Cotswolds: Oxford University, Great Hall of Christ Church College, Traditional Cotswold Villages, Bibury
So the Cotswolds tours is not all Cotswolds as you can see. At least half of it is Oxford (which to me is a plus, but may not be to you). I would sit down and look at videos or photos of each place, and figure out which you would like to see.
Honestly, I think that the Cotswolds tour might have the best of both worlds. A beautiful university city with a dizzying array of architecture and history, and then a tour of the traditional iconic villages of England. But it's your tour so enjoy whichever one you choose! Really, as the others have said, you can't go wrong with either.