Sigma 70-300 seems a little faster so better in lower light but the Tamron 28-300 is wider with internal focusing. As for what you should go for depends what you wish to photograph, will you be using mainly at the 300mm end or wider? If you are mainly at the tele end then I would choose the Sigma for its maximum aperture but if you may find 28mm useful then the Tamron is the obvious choice.
I suggest you visit a camera shop who stocks both and try them out taking a few test shots of highly detailed scenes outside then compare on your own computer back home, sleep on it and decide in the morning after looking again with a fresh pair of eyes.
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