I bought my Sigma 120-300/2.8 OIS lens last fall, literally three weeks before Sigma announced it's replacement was coming in their new lens architecture. I contacted the Canadian distributor for more information and for a month the reply was "we don't know what you are talking about" even though there was plenty of information becoming available on the web. -10 points for being obtuse. Bryan Weiss, owner of Daytripper Photo has an excellent relationship with Gentec Canada, the local distributor for Sigma lenses. They provide him samples to use on his day trips for his paying clients to use and hopefully fall in love with and buy. As Bryan is a full time seller at Henry's Camera, the client has a very simple path to follow. He graciously offered to let me try out the new Sport series of the 120-300/2.8 OIS as he had received it for a day trip.
I bought the 120-300/2.8 after using Sigma's 300/2.8 on a similar trial period based on the fact that I could buy 2.5 lenses for the purchase price of a Canon 300/2.8 and because I really did not have that kind of coin available.
The 120-300/2.8 OIS I own is a very good lens with excellent sharpness, quick AF motors and limited distortion. It's also a pig, meaning shooting it handheld is an exercise in weightlifting and carrying it around will fatigue you in due course. The colour rendition of the lens is not as good as what I find with Canon L glass, although some allowance must be made for cost and it's common that different vendor's lenses have a different colour rendition. Canon is consistent throughout the L range. Sigma probably is too throughout their DG (full frame) line, but I have no real experience with multiple lenses to provide guidance.
Here are a images of the two lenses.
The new lens increases the number of switches on the barrel to include a focus limiter to reduce focus range and ostensibly improve AF performance in poor contrast conditions. It also has a switch to invoke custom lens profiles that the owner can construct using Sigma's USB based lens adapter and software. Otherwise the switching is consistent between AF/MF and three OIS settings, although the layout is different from lens to lens.
Both lenses have tripod collars as expected and needed. The new lens has strap lugs on its collar, a useful addition. The new collar loses the finger ledges in favour of a heavier design. As I use RRS plates on my lens collars, there was no real difference to me, although the knob to lock the collar on the new lens is considerably less knurled than on mine and in hot and sweaty conditions will be more difficult to grip.
The new lens feels heavier than the one I own. Not by much, but heavier nonetheless. The filter diameter is 105mm and there is still no option for drop in filters as in the Canon line so buying that polarizer is going to feel like a mortgage payment. Sigma does have filters available for the lenses.
Balance is similar between the lenses, with acclimatization achieved reasonably quickly. Immediately after that comes the realization that you have to have at least a monopod to shoot this if your name is not Kal-El.
The Sport line has a pretty S in a silver inlay on the lens barrel. Autofocus was no faster or slower on the new lens than on the one that I own. Both lens have a bit of slop in the mount that I find extremely annoying, neither feels rock solid when mounted. In some very rudimentary shooting tests with a Canon 1Dx on high and low contrast subjects, neither lens appeared to outperform the other. Viewing shots on the computer screen, I could not see any difference between the two. Focus is not consistent across the zoom range, so focus at 120mm requires adjustment when you zoom in to 300mm and vice versa. This is not entirely uncommon so more a nuisance than a bug.
The lenses do not differ in their ability to work with Teleconverters. Sigma (understandably) specifies that to use a teleconverter, one must use only a Sigma teleconverter. This is not necessarily true as I found both the Canon 1.4x III and 2x III teleconverters to work just fine, although the Sigma branded converters will be black tubed and less expensive than the Canon white tubed versions.
I was perturbed at having spent so much money on a lens only to have it replaced in less than a month and more perturbed by the seeming ignorance out of the Canadian distributor. I am grateful to Bryan for the loaner because now I know I am not missing anything. The USB dock / profile thing is of no value to me. I can set individual lens profiles in camera and do not need to code them independently.
Should you buy one? Certainly you will save a lot of money over the Canon or Nikon 300/2.8 variants and there is real flexibility in having a zoom instead of a prime where you may not be able to control your placement and that of your subject. For OJHL hockey I find myself between about 210mm and 280mm on the 1D Mark IV and for the recent Polo for Heart the zoom really allowed for more flexibility in image capture. List price is around $3800 so not cheap by any stretch but if you need 300mm and f/2.8 this makes a credible entry point.