Sports is a pretty big category. If your talking outdoors in good sunlight then any decent telephoto lens should suffice. If your talking indoors or outdoors evening/nighttime then your going to need fast glass, F/2.8 - and anything in that range in telephoto is expensive.
For outdoors in good lighting Sony would make a good choice I would think. They have a high frame per second speed which would be nice to have in a situation like that, however the thing to keep in mind is that Sony acheives such high FPS by using translucent mirror technology. The upside is that you get very fast shooting, the downside is that because of the way the system works you have to divert some of the incoming light away from the sensor and as a result your low light performance will not match an older DSLR with the flip mirror setup that uses the same or similar sensor. So if at some point your considering shooting indoors or night games/events then Sony may not be the best option. The second drawback to Sony is that you will have more limited choices of lenses that will use autofocus and won't require an adapter of some sort, so be aware of that limitation before you purchase.