Why do we say undeletable instead of indeletable. Sometimes with other words we use an in prefix, such as with indefatigable.
I'm curious about un vs in.
An article on Mental Floss discusses and explains this: http://mentalfloss.com/article/57840/whats-difference-between-and-un
To sum it up, there is no fixed rule between using "un-" and "in-". The former usually works on words with Germanic roots, and the latter works on words with Latin roots. Making this distinction is not mandatory, but it seems to be the case.
In most cases, one prefix will be more suitable than the other. This is determined by what either prefix adds to the word: connotation, pronunciation, e.t.c. "Infinite" is a common word whereas "unfinite" is never used, though the prefixes have the same meaning. "Intouchable" is never used, whereas "untouchable" is.
Again, there is only a fine line between the two. Unfortunately, it can be extremely bothersome to differentiate. My advice is to use whichever conveys your thoughts better, and does not sound odd.