The sigma 50-500 OS was the first 'long' lens for me.
The lens is incredibly versatile and I had enjoyed it a lot.
However, It also had some points of concern, first off was the amount of dust that was inside the lens after only two weeks ( ok, safari is a pretty hostile place for zoom lenses I guess dust wise), but I could see it being a problem in the long term.
Also the Autofocus while ok-good, was a bit hit and miss at times, the increased sharpness from stopping down to f8+ meant i consistently shooting at f8-f11 most of the time.
Which can be a problem on cloudy days.
Also it was not really behaving like a 500mm lens, it was more 460-470mm (even at substantial distances)
( here is f6.3-f11 comparison, it is not the OS version , but most reviews on the 50-500 OS suggest the same thing, stopping down to f8-f11 for best sharpness)http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=374&Camera=9&Sample=0&FLI=5&API=1&LensComp=374&CameraComp=9&SampleComp=0&FLIComp=5&APIComp=4
If I had to pick a zoom lens again in that price range, I would go for the 70-300 L.
Reviews of that lens suggest good auto focus speed, great sharpness and good sealing.
It may lack a bit in reach , but you can shoot at half the ISO compared to the 50-500@f8.
I don't know which camera you use, but if it is the 18mp canon sensor, you can crop the 70-300 image to equal 450mm and be left with a 8mp image.
Pure guessing would be that 7d with 70-300 @5.6 and iso 800 cropped to 450mm, compared to a 50-500 @f8 iso 1600 shot should not be that far apart. (The better resolution and less noise on the 70-300 shot could approach the file of the 50-500).
Also the 50-500 weighs almost 2KG, where the 70-300 is 1100GR.
Here are some crops @ 300mm f5.6http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=738&Camera=453&Sample=0&FLI=4&API=1&LensComp=374&CameraComp=9&SampleComp=0&FLIComp=3&APIComp=1
So in summary , depending on what you want/need.
If reach has the highest priority the 50-500 OS is a good lens for a good price
If IQ, Af accuracy/speed, weight and sealing are more important, I personally would go for the 70-300
Btw, the 70-300 can take the 1.4 converter quite ok it seems, I don't know how autofocus impact is, but on a 5d3 , or 1d body you will still have autofocus.http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=738&Camera=453&Sample=0&FLI=5&API=1&LensComp=374&CameraComp=9&SampleComp=0&FLIComp=5&APIComp=3