I chose the Sigma apo version as the revews I read showed that it out performed the Tamron.
At 70mm it is very good
At 200mm it is still doing a good job
At 300mm it is losing the plot a little but my results vary partly because my tripod is (sorry for the language) CRAP.
The next thing on my shopping list is a new tripod.
I was at the Llangollen ballon festival this year and observed an older gent with a canon something getting rather frustrated with zoom creep on his Tamron 70-300 - he was not a happy chappy (basically each time he let the camera face down the lens elongated - this is not a problem with the sigma (or at least not my version)
The guy was a nice chap - he approached me and gave me some technique advice (correcting me), made nice somments about my camera then dissapeared - the world needs more nice people like him.
If he reads this and realises that I am referring to him - THANK YOU.