Depends how much you want to do I guess. The park gives you more opportunity to choose decorations/food etc whereas a play centre, that's all included.

I'm thinking of having DS party at Chipmunks (play centre) this year because I just can't be bothered with the set up/clean up etc and I think at his age, they'll enjoy it more.

Also depends on numbers. Play centers normally have a minimum number of children and it can get quite expensive if there are a lot of kids going...