If you don't need f2.8, the Canon EF 70-200mm f/4L IS USM Lens is better that either the Tamron or Sigma. Just to throw out another choice. Plus its price is in the same ball park (as the Tammy and Siggy f2.8) and it is a real Canon L.
EB
EOS 1D, EOS 1D MK IIn, EOS 1D MK III, EOS 1Ds MK III, EOS 1D MK IV and EOS 1DX and many lenses.
