Both forms are correct. Collectible and collectable are the alternative forms of the same word. Collectible is usually used in American English, and collectable is more common in British English. Besides the area where they are used, both words are pronounced the same way as / (kəˈlɛktəbəl) / and can act as the adjectives or nouns in the sentence. Their meanings also are the same.

Correct spelling, explanation: form more popular in American English.

Correct spelling, explanation: form more common in British English.

Definition of collectable/collectible:
Adjective: with a value for a collector;
I was always surprised by how much money people are able to pay for collectable figurines.
My grandfather left me his album with collectible stamps, I wonder if they are really worth anything.
Noun: an item suitable for collecting as an investment or a hobby;
My father liked to invest in collectibles, that is where our collection of paintings and coins comes from.
The recordings in that box are rare collectables.