If you only have time for one, take the boat. Hornblower puts you in the basin of Horseshoe Falls, surrounded by a 360-degree wall of falling water — it is the experience the postcards cannot capture.
Journey Behind the Falls is a tunnel system that lets you stand directly behind the wall of water and step onto a lower observation deck. It is more contemplative, less wet, and open year-round (Hornblower is not).
For a summer day tour, do Hornblower. For a shoulder-season tour with kids who hate being soaked, do Journey. If you have both on your itinerary, do Journey first while you are dry — the boat will leave you completely wet under the poncho.