the only thing i like the Sigma 70-300mm over the Tamron 28-300mmfor the is macro abilty. the Sigma will give you 1:2 at a comfortable distance of about a meter(i think...might be less). it makes taking pictures like the one below pretty easy =]
I can focus at less than a meter with the Tamron, actually down to about 40cm at the full 300mm, resulting in a 1:2.9 magnification factor. At 1 meter it has the same 1:2 factor. So, it wins in that regard, too.