Need help deciding on a lens

Basically this lens will be used for sports to shoot for my colleges paper as well as any other telephoto lens opportunities. This will be used for college indoor basketball (with a flash) which i can get pretty much under the net, college baseball (can get in the dugouts), college lacrosse (can get to anywhere against the fence but that's the closest), college softball (no one cares where i stand haha), and probably track.

Anyway my choices are:
-Canon 70-200 f/4 L
or
-Sigma 70-200 f/2.8 HSM II (Suggested by two members of the forum)

Anyway i'd love everyone's .02 . Photos with the lenses would be absolutely great. Please help me out since this will be the first actual quality glass ill be buying

Plenty of info on both lenses here and in the archive. the f/4 will not work in a gym. And the 2.8 will may not suffice for basketball. Many shoot basketball with the 85 1.8.

The f4 is a great lens. I have a Sigma 2.8 and love it. It all depends on how much light you are going to have when shooting. For me, I would rather have the extra stops.

My schools gym is actually not HORRIBLY lit. Believe it or not ive used my 75-300 garbage with a flash on my hot shoe pointing just under 90 degrees and the lighting has actually been sufficient. My friend also uses a 70-200 f/4 L with a flash and his photos look stunning. Might just be lucky our gym lights dont totally suck

Ill ALWAYS have a flash on me when i feel lighting would be a concern so really im wondering how the sigma compares to the canon in IQ i suppose

Canon is more visible to some extent, especially wide open, and Sigma is a little warm, while the law is neutral, and that I love. Canon built the best, but Sigma will hold up to most conditions the average photographer will encounter. The other thing is that Sigma and sometimes have problems communicating with Canon cameras. I have not seen anything about any problems with the 10D. Sigma I had to do not work on my D60. Heard in some forum that the person contact Sigma about this problem and said it costs about 50 dollars to correct. But all in all, I think that Sigma is a heck of a lot of lens for $ consumer.

peoples' constant complaints about sigma's focusing issues also has me worried. This telephoto decision is legit one of the hardest things ive ever had to think about haha