The former seems more natural to me, and is personally what I've used, but the latter is easier to pronounce.
Pronunciable is incorrect.
The correct one is Pronounceable which means capable of being distinctly pronounced in speech; enunciable.
Pronounce (verb) + -able (suffix) -> Pronounceable (adjective).
We often add the suffix -able to verbs to make adjectives (Read this)
Examples: Enjoy (verb) + -able (suffix) -> enjoyable (adjective).
Do (verb) + -able (suffix) -> doable (adjective) etc.
Pronunciable seems natural to you because you think it resembles pronunciation (noun).
Pronunciation of pronounceable is not the same as that of pronunciation, it's simply pronounce + -able (/prəˈnaʊnsəb(ə)l/).