I think both spellings are becoming acceptable worldwide. I work part-time for the British Council in China marking IELTS (International English Language Testing System) exams and both s and z spellings are considered correct on the exam. I agree that what's important is being consistent.
Until recently I didn't realize that throwing 'u' around isn't the way to make it Canadian. I read somewhere that colour expands to colorful, humour to humorous, not colourful or humourous. Whadda youze guys say, izzat aboot right?