I personally find two distinct senses for each word. If we have to pick one or the other, the one I would choose would fit in as many different ways as possible.
- great; superlative.
- relating to, or having the qualities of, fantasy in general or one
fantasy in particular.
"The idea that the day the last American finally buys a gun, the whole country will turn into one big cool-headed, peace-loving family has the air of the fantastic."
- being or relating to fantasy or some particular aspect of fantasy.
- fantastic, but with an extra syllable for clarity or emphasis (Edit: much as people do with "ironical" ).
"UFOs do not really exist, but if they did, their alien passengers would not cast magical spells. Spielberg's E.T. was less science fictional than it was fantastical."
Either one could be fine. Which one works best I think depends entirely on context.
: E.g., Robin Williams' character, Sean Maguire, in Good Will Hunting:
SEAN: Why don't you come, I'll buy you a drink.
LAMBEAU: The drinks at those things are free.
SEAN: I know, Gerry. I was being ironical.