Only one of these, extensible, was a word at all until the 1940s[0], and extendable is still the less common word.
This is the not-uncommon case where the irregular form is slowly losing ground to the regular form. It's possible that in a century, 'extensible' will be considered archaic.
This is the not-uncommon case where the irregular form is slowly losing ground to the regular form. It's possible that in a century, 'extensible' will be considered archaic.
[0] https://books.google.com/ngrams/graph?content=extendable%2C+...