Very good question! In the UK, a lot of the time you will IS-YOO. In Canada, you will hear both IS-YOO and ISH-OO; however, ISH-OO is more popular. I believe ISH-OO is also more popular in Australia.
It is important to remember that both pronunciations are equally acceptable and widely used worldwide. Local varieties of English will also differ. In other words, it doesn't really matter which one you choose. Hope that helps :)