Is there a preference between 'submissible' and 'submittable'?

If you are a legal writer, use submissible.

You will find both in a dictionary like the Oxford English Dictionary (submittable, submissible) to mean "capable of being submitted"; the only significant difference is that submissible can also mean

(Law) able to be submitted as legal evidence,

a usage supported by legal dictionaries (Merriam-Webster).

If you prefer a prescriptivist standard, use submissible.

For example, Bryan Garner marks submittable as a "needless variant" in his book Garner's Modern English Usage (under submissible), a phrase that means the second spelling has no significant difference in meaning and thus should be avoided. While the variant is "needless," Garner notes that the ratio of submissible vs. submittable in World English is about 5:1.

Otherwise, use either.

As noted in the Oxford English Dictionary, both words are in current use.

At least in an Ngram, submissible has been on a long and slow decline since the 1950s, at least in Google's corpus, though only recently have they been anywhere close to parity. The chart below is a general corpus search, and searching the American and British corpuses separately didn't yield significant differences (Ngram):