I can vouch for the Sigma 70-200 2.8 EX HSM II. Great lens, lovely IQ, sharp quick to focus, great colour rendition and built like a tank. I use it at indoor showjumping at f2.8 and it performs perfectly.
Its as good as the Canon IMO and give you the extra stop of light.
Not used the Tamron but I do see Jessops have 40% off Tamron lenses at the moment!
Had a look at jessops, could not find a discount.
Ray