If your question were about the *-our/-or* distinction, or the *-re/-er* distinction, it would be a real question, but with *-ise/-ize* there's no contest: go with the Z form, as it's [accepted on both sides of the Atlantic][1].

The International Organization for Standardization favours British spellings, and uses the *-ize* form. (This is known as [Oxford Spelling][2].)


  [1]: https://english.stackexchange.com/a/4063/1696
  [2]: https://english.stackexchange.com/a/77246/1696