I know this guy, really well, who USED to have a Sigma 120-300 f2.8 that he sold a year ago. And I also know that this same person has regretted the sale ever since. I also know, really well, the fellow who bought the lens from this Rex fellow in this thread, in fact I might go so far as to say that he is sitting right here typing away, moving my fat fingers as it were :biggrin: And if you were to ask me flat out who this is, well, I guess I'd have to admit it is ME :biggrin:
As with the Sigma 500 that I had, and that folks like Jim Thiel are currently using, I'd rate the focus speed as just a touch, maybe, slower than the equivalent Nikon. I don't think you are going to see "night and day" differences. Off course if the 300 is the VR version, you will get that added bonus.
As to the TC's, I could only see one advantage to using the Sigma TC's vs. my converted Nikon TC's, and that had to do with recording of the EXIF data. With the Nikon TC's the recorded focal length and aperture are not multiplied, but the metering is all correct.
My opinion, it must be a great lens or why would I be stupid enough to buy one again? And it AIN't just because Rex is a nice guy :biggrin:. And for me, shooting youth sports, the zoom far outweighs the slight AF advantage I would see from the Nikon 300 AFS. Question now is, do I sell my 70-200 AFS f2.8 VR?