It is or it's?

I was thinking about something, which is better to use in a story it's or it is? What would you prefer? :)

Thanks!

Depends on the character. If he or she is very proper, and frowns upon conjoining words then 'It is' would suit it.

But 'it's' is more commonly used.

@swedishfan
Then you have it's and its.

Just remember it's = it is or it has
Like ~ It's past my bed time or It's really dark outside.

While its is for a possessive.
Like ~ Every dog has its day.

* just remember; that if you can replace it with the word his or her; there's no apostrophe (: