Hoping for some comparative reviews of these two lenses... I have chosen these two as they are in my price bracket...

Sigma Lens 70-200mm f/2.8II APO EX DG HSM Macro
Canon Lens 70-200mm EF f/4L USM

The Sigma is about $300 more expensive at the place I usually get my stuff from (though they retail about the same i think). I have read a lot of good reviews of the Canon's "L" glass... high contrast, sharp, etc.
How do the optics compare? and... Will the faster lens be worth the extra $300, or are the optics on the Canon superior despite the lower max aperture, and hence I should save myself $300?!

Type of shots... general purpose, wildlife, sports, etc.

Any help woul dbe greatly appreciated!
Thanks
Ash