Sigma 100-300 f/4 EX HSM...make sure it actually will FOCUS reliably with whatever Nikon you have. If you have a "new" Nikon, there's a decent chance the lens will not focus correctly, since many of these are
older lenses, and with the Nikon D200, Nikon severely "broke" the AF-S focusing protocol for a lot of older, third-party lenses. Just as Nikon encrypted RAW white balance information beginning with the D2x, so Adobe software can NOT read the WB data, but only "guess" at it, so too with a lot of older, third party lenses that use an "imitation AF-S" protocol, like Sigma's HSM protocol...my 100-300 f/4 EX HSM and 180 APO-Macro EX HSM Sigma both had terrible problems with D2x and newer class Nikon bodies. In other words, make sure you get a NEWER version of the 100-300, not an oldie...
Nikon does not provide independent lens makers with any information on anything...those companies have to reverse engineer everything...and if you want automatic in-camera chromatic aberration correction on JPEG files...and so on???? Better make sure it's a Nikkor lens on your newer Nikon camera. I'm not saying the 100-300 is a bad lens...just that there is a chance that there "could be" issues with older lenses. I have not tried the Sigma 100-300/4 EX HSM that I own on any of the very-newest bodies, but Nikon DID change the autofocus switch system located on the body fairly recently. I am not sure if "older" Sigma lenses can be re-chipped to bring them up to compliance with the very-newest Nikon bodies. My problems with the 100-300 and the 180 EX HSM MAcro were with weird focus hunting behaviors, under relatively "normal", everyday shooting situations...the lenses would just...bug out..and would radically mis-focus, often on relatively normal, ordinarily easy to focus subject matter. For sports, the 100-300's tendency to back focus by 10,15,or even 20 meters distance made it simply impossible to rely upon in autofocus mode.
Here is a test result that matches my experience with the 100-300; possibly a decentered element. One side of the lens performing poorly.
Sigma Lens: Zooms - Sigma 100-300mm f/4 EX DG HSM APO (Tested) - SLRgear.com!
As they note, the 70-300 VR Nikkor is sharper, and has more CA, and does not have the constant aperture...but then again, you can find 70-300 VF's for, well...I payed $349 used for mine... I hate to rain on somebody's lens buying parade, but my experience with Sigma has not been without quality control issues and odd performance issues.