While I agree that the Canadian side is best, don't worry if you can't do it. The American side is well worth the trip on its own.
The Canadian side has better views of the Falls as a whole, but the American side is much more natural a setting when you get to Goat Island and the Three Sisters Islands. You can walk right to the edge of the water (BUT PLEASE pay attention to the warning signs and DO NOT wade in the water there - the water is deceiving and can sweep you away very quickly and you really DON'T want to go over the falls, especially the American Falls).