The shorter guy will be harder, he can get low and hit you at speed so if you don't get low enough you'll get knocked out (speaking from experience), the taller guy can't get as low and have as much speed on him so if you hit him at the knees or lower he will go over the top of you and hit the ground like a bag of bricks :)
From experience I'd say the latter (shorter guy). Short heavy guys have a lower center of gravity, Which basically means their going to be more stable. But theirs such minute difference it'd it really depend on the skill of the tackler.
