Sigma 120 400 vs sigma 120 300 os 2012 + 1.4tc

[alexturton]

Btw, here are some fantastically sharp shots done with a D4, via the 120-400 Sigma lens I'm considering purchasing.  (He also has others done with a D700 that look about as good). 

http://www.pbase.com/jeff1959/nikon_d4_images_july_2012

these shots are in direct sunlight @ f9. Most lenses are sharp by then.

don't get me wrong, the sigma is a sharp enough lens when you can stop down (around f7.1 - f8), and the subject is big enough (and slow enough) for the AF to track. But when the light is poor and the subject is smaller and moves quickly (like a small garden bird) its just not good enough.

[CarlTN]

Indeed, but my point with posting those, was that it kind of flies in the face of those reviews that say the 120-400 is very soft.  Obviously there are bad samples that are, so I hope I don't end up with one.

I'm not sure I agree that all lenses get sharper by f/9.  My 135 f/2 doesn't, and neither does my 70-200 f/4.  By f/5.6 yes, but by f/8 there's a tad of softness. 

Well, you are ready to move up to a large supertelephoto, and are ready to spend $3000 to $6000 or more.  I am not.  I've tried them.  I'm comfortable where I'm at right now.  I'm happy to just shoot larger targets in low light with my 135 f/2 (such as deer, raccoons, foxes, water fowl) and continue to save for a supertele.  I have other monetary commitments right now.

The best you could do for low light birding, would be the 1Dx with the 400 f/2.8 ii or 600mm f/4 ii.  If you need a low light zoom, the Sigma 120-300 f/2.8 2013 version is probably it.  Supposedly it will be released mid March.