Try both and you'll see for yourself. However I can tell you Kyokushin is known for its really hard physical conditioning and out of all the styles I've trained in and dabbed in, Kyokushin is up there in terms of the hardest conditioning. It's not as much of a focus in Tae Kwon Do considering the art and competition formats.
Kyokushin is a lot about developing strength and being able to go through damage and pain, hence the hardcore conditioning. Tae Kwon Do is more about flexibility and speed (even though some dojangs do some conditioning too). That's a very simple way to look at it.