Sealion have you visited Marwell before? If not maybe you should decide on your first visit whether to buy a membership or not (a lot of them will take your entrance fee off the membership when you join on the day - not sure about marwell)
Also i would look at bus timetables and cost before committing, to get to Colchester zoo from my house by car takes approx 30-40 mins by bus over 2 hours just to get there (due to schedules) and costs approx £12!! (return)
You don't have to defend being a member of Colchester mazfc

when we were renewing our platinuim membership recently they assumed my partner (who lives 3 hours from me!) was joining as it worked out cheaper than other zoos nearer to him! till we pointed out this is his regular zoo

Like you we use the restuarants/gift shops at the other zoos we visit and in the 4 weeks since renewing ours we have been to Colchester twice, Chester and Bristol