maybe because we are spoilt
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I find it fascinating that an upgrade on the Sigma 120-300mm f/2.8 DG OS HSM has happened so quickly. The previous model of the Sigma 120-300mm f/2.8 DG OS HSM was almost there for me, just lacking very slightly in some key performance areas.
As people are saying "Sigma do listen", there's a big chance I'll pick up this lens. If the required performance is there, my EF 300 f/2.8is could be headed for retirement. Think about it, a 120-300 f/2.8 zoom makes a pretty compelling case, especially if the critical areas of IQ and AF speed and consistent build quality cross into premium territory.
IS is simply a must. The difference in terms of the keeper rate is considerable.
Or look at it this way. He wants it for sports mainly.
Well, in bright lighting sports, the f/2.8 would let you use a faster shutter, and gaining one stop of aperature (f/2.8 vs. f/4) lets you advance one stop in shutter speed (double speed), making up for one stop of image stabilization. Does the f/4 IS really give you much more than 1 stop of IS? And if it is really bright, then forget about IS completely. At a very fast shutter speed (like bright daylight fast sports), Image Stabilization doesn't really do any good. So in some situations at medium speed, the two may cancel each other out, and in others at high speed the IS is useless.
And in low lighting, like in a basketball gym, the f/4 may not be usable at all (unless you have an uber-expensive camera that is great at uber-uber-high ISO), whereas the f/2.8 is twice as bright as f/4. And since you would want to use a monopod for that anyway, the monopod will be your image stabilization.