I don't know but Americans have a special attachment towards z, they frequently change spelling of words with s to z! Don't ask me why!!!

Both are Standard English as @sinchona said..

But this is a matter of your locale, it would depend on where you are residing. British use 's' in specialised while Americans use 'z' instead..

Regards,

I don't know but Americans have a special attachment towards z, they frequently change spelling of words with s to z! Don't ask me why!!!

Both are Standard English as @sinchona said..

But this is a matter of your locale, it would depend on where you are residing. British use 's' in specialised while Americans use 'z' instead..