These are the same word, and most dictionaries accept both spellings—although your spell-checker may disagree!
Collectible is considered more correct by many people. The difference is that collectible is considered its own word, beyond being derived from the verb collect, whereas, collectable is merely the verb collect with -able added on—in the way that we can add -able to most English verbs if we want to. Therefore, some people say that collectable means only “able to be collected” while a collectible is more specific: collectibles are items made and sold specifically for collectors (people who collect things for a hobby), such as dolls, comic books, and other small works of art:
People used to be proud of their music collections, but online music is not really collectable.
The signed first edition of this book is sure to become a valuable collectible.
However, both spellings can be used with this meaning, though different English speakers may disagree.