The Cave of the Winds, located on the American side, starts with an elevator ride down approx. 53 metres / 175 feet into the Niagara Gorge to a series of wooden stairs and walkways leading you to within 6 metres / 20 feet of the Bridal Veil Falls.
The Journey Behind the Falls, located on the Canadian side, starts with an elevator ride down approx. 45.72 metres / 150 feet through the bedrock that forms the Horseshoe Falls, to tunnels that will lead you to an area about 1/3 of the way behind the Falls. There are a number of lookout portals where you can watch as the torrents of water fall at approx. 40 Mph / 65 Km/H. Also, there are two observation decks that allow you to stand beside the foot of the Horseshoe Falls.
While the two sound quite similar, the experiences are quite different. If you can, I would suggest experiencing both !
p.s. Cave of the Winds is a bit of a misnomer, as there is no “cave”.
Also would recommend trying both if you have the time. If not, I think Cave of the Winds is the best attraction anywhere in the falls. You can literally feel the power of the falls rushing down as you stand almost right under them.