In my own idiolect, "fantastic" can mean "having fantasy elements" or, metaphorically, "very good," while "fantastical" can only have the literal sense. So, for instance, a fairy tale might be "fantastic" or "fantastical," with the latter likely being preferred, but a very good meal could only be described as "fantastic."
To my surprise, dictionary.com does not agree with me, simply listing "fantastical" as a variant form of "fantastic."
This doesn't seem right to me. Are there people who say "last night's meal was simply fantastical"? Is the distinction generational and not reflected by dictionaries? Is my usage pattern idiosyncratic and not well attested by other sources? In short, do most English speakers, or most English speakers of a certain age or region, distinguish between these words, or are they entirely interchangeable?