Both have their strong and weak points, most of which have been pointed out by previous posts. For 12 years or so we ran 2 boats, a 24 ft racer with a tiller and a 40 foot racer/cruiser with a 60" wheel. Switching back and forth became second nature after a while. While I don't really have a strong preference as far as the actual steering is concerned, I did find that a cockpit that is designed around a good wheel installation offers more variety in comfortable seated/standing steering positions for extended trips (i.e. 6 - 8 hour days).
The wheel-steered boat had a chain and cable/quadrant setup and was just 1 turn lock-to-lock so it was very responsive and direct with reasonable feel. The large diameter allowed the helmsman to sit well outboard with good sightlines for waves and action on deck. The small "McGregor" wheels you see in smaller boats don't make much sense to me - lacking mechanical advantage and keep the driver in the middle of the boat. In those cases a tiller would be a vast improvement.
Our current 35' boat has a wheel with rod and lever connections, and nearly two turns lock to lock, does not have as much "feel" as the old one. Wheel vs tiller was not part of our decision-making when shopping for this last boat. Other factors were more important to us.