The /i/ looks awful. I am amazed they're both A-OK with Webster.

From what I can tell, the OED prefers includible to includable (calling the second a variant of the first). Excludable is listed, excludible is not.

Although this seems to be sitting on the fence, my guess (from the citations listed) is that /excludable/ seems to be chiefy American and /includible/ seems to be chiefly British.

Wow, now this is going to bother me as well. I always use the /a/ format, and I agree with Craig, the /i/ has a very British feel to it.

For what it's worth: American courts usually use the /a/ version (generally in the context of whether a certain period of time is "excludable" under the Speedy Trial Act).

If you want to do this democratically instead of through the elitism of dictionaries, Google gives 33,500 results for includible, 28,400 for includable; 3,510 results for excludible plus a "Did you mean: excludable," 75,800 for excludable.

As usual, the people are inconsistent and don't know what they want.

these are hard for me for two reasons. growing up in delmarva, all unaccented sylables are given the schwa sound, so -able sounds like -ible.
and i read a lot of british english, without knowing it has slightly different rules, so i'm fuzzy on the variants.

There is little logic in it, as far as I know, the preference between the -ible/-able suffix is not restricted by any sort of spelling rule (I may be wrong).

It stems from borrowing from Latin, mainly. See http://www.bartleby.com/68/19/19.html for a little blurb.