Tamron, Sigma and Sony lenses

[hunker7]

hello! I have Sony Alpha A230 and i thinking about 70-300mm lens. And i wanted to ask, what`s the difference betweed Tamron, Sigma and Sony. Sony costs about 170 quid but Tamron and Sigma about 130 quid. so, is it better to buy Original Sony lens, or there is no diference.

Thanks