Either lens will work fine. You may find that you use the wider angle lens in the main chamber, but I think you'll put the longer lens on when you are in the narrower parts of the canyon to isolate the formations from the rim. Getting the rim in most photos will only ruin them. I would leave the 50mm at home. I find that on my crop camera, I am using a 20-40mm length lens. Using a tripod is the only way to get a decent photo at Antelope. I have tried HDR photos and enjoy the results I get from this technique, but it isn't necessary.
My method for shooting at Antelope is to find an interesting formation that is getting reflected light in the heart of the beast. By that I mean try to stay away from the top of the canyon. This is where a medium telephoto comes in handy. The light picks up color from the walls of the canyon each time it bounces off of the wall. You will find the gold colors higher up, reds in the middle where the reflected light strikes the walls and it turns a bluish black at the bottom. Look for spots that have all three colors. I find that having a dark shadow only enhances the colors of the canyon so I try to have some in the photos. Pay attention to the texture of the rock. It really adds interest to your photos.
I like to meter for the mid-tone reds. This lets the yellows and darker colors come out. A habit I got into during my film days is to take long exposures, sometimes up to 30 seconds. For some reason, this lets the colors really develop on film, even taking into account reciprocity. I still do it with digital and find the results are more than satisfactory. I've taken shorter exposures but I still find exposures of 5 seconds or so more pleasing to me. One advantage to a long exposure is that anyone walking in front of the camera disappears, and there is almost always someone walking in front of you, especially if a group of Japanese tourists visit at the same time. They are most pleasant as they walk in front of your shot, but they won't stop. Europeans tend to walk in front also, but they don't apologize. If someone stops and asks if you are through so they can go by, you can bet they are from the states.
As for the beam of light, don't worry about it for another three months. It is only found in the upper canyon from about the middle of March until mid-September. If there isn't a beam, the guides won't be throwing any dust. The dust lets you see the sun beam. If it is windy outside, sand will be blowing in, however. I usually take something like a boonie hat and place it over the camera whenever I'm not shooting to keep most of the dust and sand out.
One thing to keep in mind is that the canyon is small, less than a 1/4 mile in length. The lower canyon is twice that. If there is a chance for rain, don't even think of going. Change plans to something safer.
Laurent Martres writes an excellent series of books for photographers covering this area. His older book "Land of the Canyons" would help you here as well as the Grand Canyon if you can find it. Highly recommended.
Have fun and take lots of pictures.
This was my first time & frankly the next time I would like to do it on a light overcast day instead of full sun. Try to concentrate on indirect light & not try to get the sky in unless there are nice clouds .... just my opinion. Also I will use the tipod, but only for about 6 shots.
Photo tour durring the busy days at lower antelope would be a wise choice .... allows you to have your own crossing guard 
