They are both correct. US English tends to prefer the 'z' spelling, British English tends to prefer the 's' spelling, although there is some overlap. If you want to upset fewer people and fewer automatic spell checks, you may as well use the 'z' spelling, as this is more common. Personally, I always use the 's', but the choice is yours.

Here, in the US, we usually write those with a "Z" Both are correct. Remember, languages are very local. We have words in Texas that are not the same in New York, but all are correct. Just yesterday, I used Spanish word for basketball = baloncesto. My Spanish teacher did not understand what that word meant. They use a different word for basketball in her country. (Argentina), You are doing well in writing English.