If you can only do one of these, I would do Lower Antelope Canyon. Why?
Antelope Canyon is an amazing slot canyon near Page Arizona on Navajo Tribal Land. During the off season, the people at Lower Antelope Canyon will let me go on my own for 2 hours. It’s called the photographers permit. It’s about $35. The Lower is about a mile long and you can walk to the entrance in a minute. At the Upper, you have to be driven out to the trailhead and be guided through the canyon which is only a 1/2 mile at most.
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The other thing I don’t like about The Upper is that you have to return the same way you came in. That means bumping into all the people going in to the canyon. That can be a problem when you have that perfect shot lined up and people get in the way. And then you’re probably in some else’s way. In the Lower Canyon, you enter in one spot and exit at the other end. Usually everyone is moving in the same direction.
If you’re claustrophobic, then maybe the Lower isn’t for you. You have some tight spots to squeeze through and ladders to negotiate.
If you are going to seriously photograph Antelope Canyon, don’t go during the busy season from Mid-March through October because you won’t have fun as the guides will rush you through. Although this is the season you are more likely to see the beams of sunlight at midday. And remember to shoot with a tripod and bracket all of your shots. You don’t want to ruin this opportunity.