sony a77 should I or shouldn't I?

I am seriously considering purchasing the sony a-77 with kit lens plus a 70-200 f2.8 lens. I still am unsure if I should go sony or nikon. I am a serious enthusiast. I shoot mostly sports photography and often in low light settings. I also shoot location, natural light portraits. The latter I believe the sony a-77 will do fine with. It is the low light, action performance I am concerned with. I read a lot of chatter about more noise at higher ISO (above 1600) on the sony as well.

Also, which lens performs better? Sigma or Tamron 70-200?

If I am

What are your thoughts?