According to Wikipedia (that fount of secondary knowledge), the OED lists both collectible and collectable as equally valid spellings.
So, on the faith of this graph (which shows percentages of the corpus, not absolute counts), neither was of much note until around the late 1820s (collectable having a slight edge in the teens). Collectable then held the lion’s share until around 1875 and thereafter collectible has stolen the show.
This suggests that neither is “correct”, but that collectible is certainly more popular now (by a factor of almost 3x) – at least in Google’s corpus.
Ah… restricting the searches to British/American English corpora indicates that collectable is a British preference, and collectible is a (strongly) US one (first British corpus):
And that Americans, apparently, had an earlier interest in collectables than the Brits did.
Now I am somewhat troubled by the Proof by Google that I’ve just stumbled upon.
PS: similar results are obtained if you restrict it to noun usages only