Khiry has a much better first touch, probably has better ball skills, but isn't yet as good a finisher or overall player as Zardes. Both players are young (22 and 24) and will improve, but my bet is that Khiry will improve more. He is younger and even farther back in development, and my gut tells me he will be the better player.
Zardes has had some good moments, but I thought he played particularly poorly in the last game. Not just his heavy touch, but his positioning and judgment were sub par. There were even a couple of moments on defense that were bad, including one where he failed to mark a runner near the box who was then put through on goal.
It's worth remembering that Zardes is probably 5th in the USA's heirarchy of forwards. He's only on the field because Altidore and Johannsson are hurt - if either of those guys were healthy, Zardes might not even be on the team. Zardes has also been starting because we are playing a 4-3-3, so we need 3 forwards and his speed makes it easier for him to play the wing, where a guy like Wondo can't really play. Frankly, I would put Zusi in at wing for the next game.