This is mostly a matter of distinguishing between having worth ("all its worth") and being worth something ("all it's worth"). I can only find subjective arguments for the value of either case over the other.
Interestingly, it appears that the it's form is at least slightly more grammatically correct, per this answer to a similar question. To paraphrase, the word "of" is necessary: it should be "all of its worth", because its is a (possessive) pronoun and must therefore be preceded by "of". Therefore all its worth is actually wrong.