Definitely heartless. A brainless one wouldn't learn from mistakes. Plus, cruel bastards usually get along with I.
I simply wouldn't go on the picnic unless the person was neither, or would that be heartless of me???
I'd rather be around someone who is brainless than heartless for the same reasons Ol_hippie and psyche have given
I think I would have to say brainless as at least then if there was not conversation that was stimulating if they had heart we could still enjoy the whole concept of a picnic and the beauty of nature. What a choice to have to make though.