The cat and the peanut butter sandwich would hit sideways.
The cat would explode on impact, then because of the greatly reduced weight (and possibly because the string will break), the peanut butter will then fall over, resting buttered side down, thus ensuring the greatest mess possible.
Of course, I could be wrong.
The cat could land on its feet, but all it's bones in its legs would break, ensuring that the rest of it explodes on impact, and then the peanut butter sandwich gets ripped apart too, all pieces eventually landing buttered side down.
Murphy was a bastard. I like cats.