You can change only one. Personally I would have a bigger garden. I spend every walking hour out there when its summer and to have it a lot bigger would be fantastic.Plus I would have room for a shed! ( to lock hubbie in! -joke!)
we have half an acre + and it is hard work keeping it round. that said i wouldnt change it. The house has 4 large bedrooms which is nice as it means grandchildren and all the family can comfortably visit. The time will come where the garden is too much for us i am sure- but not for a year or two yet!
Bigger house, I'm perfectly happy with our garden which is massive but we have a downstairs patio area which we maintain and the upstairs flat keeps the big garden looking beautiful for us to sit and stare at from our little patio area... it's the perfect arrangement :c)