I would vote for the D200 also. Lower res but is better protected. Lower res is still fine for any screen display, just not as good for large prints At a distance, where the air is clearer of mist, higher res is a good thing but close to the falls, the air is filled with diffusing water droplets so the contrast is poor and detail is low. Light is good so the weaker ISO performance of the D200 will not be a factor. The mist gets into everything, more than rain because the droplets of mist are so small they float in the air for hours because joining other droplets and falling as rain or evaporating if at the top of the air column. I was hiking up a fall, tiny compared to Niagra but the mist was similar and everything had water inside when I stopped for the night at the top of the 3000fit climb next to the falls. Many rain resistant backpackers wallet, inside my pack, turned my few dollar bills into moist limp pieces of blotter paper that took all day laying on a rock in the partital sun to dry out. My down sleeping bag was damp even in its rain resistant drawstring bag. I am not sure a raincoat is going to help, raindrops are 1000 times larger than mist droplets and predictably fall towards earth. They behave differently and mist has no direction other than air current, gravity has a lot less effect on them than the fall's wind currents.