I've heard some good things about the new Tamron 600mm lens.
I have had both the bigma and the 300mm. I felt they were both fair lenses for the money but mine were a little soft at the long end, which isn't unusual. The issue I have with both sigma and tamron is quality control. Copies can often vary quite dramatically. That isn't to say the same isn't true with other manufacturers, it's just personally I have found this to be more so with the Sigma and Tamron. Of course you could easily argue that this is why they are generally cheaper.
I ditched both and saved up for the Sony 70-400mm, and never looked back.... Hope this helps.
P.s. What is it you want to capture btw, as this could have some bearing? It might be worth thinking about a 70-200 f2.8 and a 2x extender. The new ones are not cheap, but you could probably get a second hand one on ebay. I am not a big fan of extenders myself, especially the 2x, however I cannot deny that some of the best photos I have taken has been with a 70-200mm lens. That set up is really flexible too if travelling.