I've been to both and I've honestly never even thought about comparing them. They are two dramatically different places, and it's truly bizarre to me that anyone would even try to compare the two. I'll even say, Niagara Falls is perfectly fine, but it isn't even on my personal top 10 of favorite waterfalls, but that has nothing to do with the Grand Canyon.
Maybe more to the point, if you don't think you'd be able to enjoy Niagara Falls after seeing the Grand Canyon, then what places on earth could you enjoy after seeing the Grand Canyon? Would you be able to enjoy the waterfalls at Yosemite? Would you be able to enjoy the California Coast? Would you be able to enjoy Sequoia, or any other place you wanted to see out west?
If you don't think you'd be able to enjoy a natural wonder like Niagara Falls, after seeing the Grand Canyon, then maybe you need to wait until you're a senior citizen to see the Grand Canyon, just to make sure every other natural wonder you see over the next several decades isn't "spoiled" because it won't compare to the Grand Canyon (that's hyperbole, but hopefully it illustrates how silly that ends up sounding, and will get you to adjust your thinking a bit.)