Quote: And be aware of the fact that there are several different versions of bigmas. 50-500 EX is the best one, 50-500 OS is probably second, 150-500 EX is just ok and 170-500 is the worst.
I have had the Bigma EX and now own the OS. The latest version with OS has much better IQ than the EX.
If you are needing a lens for wildlife, then the Sigma 50-500mm OS will be the most versatile and the IQ at 300mm is extremely good. I also have the DA* 60-250 and the DA*300mm. I use the two lenses for very different purposes. The 60-250 actually replaced my 50-135 and it works great as a portrait lens while giving me a high quality lens for travel. It and the AFA 1.7x will get you to 425mm in a relatively light compact kit. I pair it with my 16-50 as a travel kit when I think that I might get some wildlife shots.
The DA*300 is a fantastic lens also that is relatively light and produces great IQ. It works well with the AFA 1.7x giving you a 510mm lens combo in a small package compared to the Bigma. I found that the DA*300 with teleconverter actually gave as good or better results than the Bigma EX.
I understand your wanting to explore different lenses than the 150-500mm. (which by the way handles similar to the Bigma) That is why I have all 3 lenses along with a Tamron SP 400mm f4. But that is another story.
You say you are into birding and wildlife. I would rule out the 60-250 for that intended purpose and go for the 300mm or the Bigma. If you go the 300mm route, a teleconverter should probably be considered if you are planning to photograph small birds.
Good luck with your decision.