I have the Minolta 135mm 2.8 and LOVE that lens. It's one of my favorites! I find it to be quite sharp, excellent colors and nice bokeh and it's so compact. I did use it to shoot at my nieces gymnastics meet and had some good luck with it. Something with an even larger aperature would be even better as gym lighting can be so difficult but for under $500 all that I can think of will probably have too short of focal length.
I've also heard good things about the Sigma 105. I don't have that but my best guess is that it will be similar in auto focus speed and sharpness of the 135 (did I mention that I LOVE the 135?!?)

The 135 is not the fastest focusing, but it's not terrible either and in my limited experience with macro lenses they are not built for speed. But most likely it will not be a problem with the competition shots.
In deciding I'd consider the focal length - will it help to have the added length of the 135 or can you get close so that isn't a factor? Another thing I'd consider is which I would rather have later - a macro lens, or a compact 135mm.
Another option would be to rent a lens for the competition. I know of a place I could recommend that has the 135mm for under $20 for one week rental. And if you did decide to go the rental route there are other lens options as well that you might decide would work even better for the occasion like a 200mm 2.8 or 70-200mm 2.8 or even a 135mm 1.8. It would depend on how important it is to get the best pics possible at the cheer competition or if you are looking for a lens to buy and the cheer competition will be it's first use?