I think it's all pretty much the same at these levels of optics.

I'd say go with the Siggy 100-300/4 and a 1.4x TC. I think Andrew(I@M) has this combo and is happy with it.

This way you get a relatively fast 300mm for when you want that, and then a standard-ish 420mm f/5.6 when you need reach.
I think that either way you'll be shooting in the f/6.1-f/10 range with some birds anyhow, simply for the DOF and contrast(sharpness).

The benefit of having the Sigma 150-500 is the OS to make it easier to handhold for when the tripod/monopod becomes an annoyance.

Unless you really want the extra width of the 50-500 over something like a 150-500, I'd stick to lower zoom ratio lenses