I agree with Sulhan about the optical quality.. (contrast and sharpness perhaps?) between the two lenses. But there are still different between them, and whether the difference justify for the price different..
1) Its a Minolta. 3rd is usually priced lower than main manufacturers, and keep in mind that main manufacturers are not responsible for compatilibly (and future) of your 3rd party lens. So that means, you investment
may become worthless suddenly should Minolta change its technology. My advise: buy 3rd party for what you need now, not as long term asset.
2) Circular Blades. Although I've not direct comparison between the two, but Minolta circular blades is known to produce pleasing bokeh. I've seen one test done with Carl Zesis, Canon, Minolta 85mm.. not hard for us to guess which one is minolta (not surprise others guess wrongly).. :bsmilie:
70-200 SSM spec
http://kmpi.konicaminolta.us/eprise/main/kmpi/content/cam/cam_attachments/AF_70-200mm_Specs
3) Minimum focus distance of 1.2m (KM) compared to 1.8m (S). is this useful to you? I not sure.. my 70-210/4 can focus about that distance.. I'm sure I've taken shots at this distance with 70mm focal lenght.
4) Additional features : Focus-range limiter,* Three focus-hold buttons,* AF/MF switch,*. From my own experience, I find that focus-range limiter can be very useful to speed up the AF in many situation. And focus-hold button (also can be DOF preview), comes very handy too, it is more convenient than reaching for the DOF button on body when hand on the grip (somemore this lens got 3!!)
5) Two DMF modes* (KM). Those who use Canon USM (and Nikon silent wave) will know this. The lens allows you to override the AF anytime when it is doing the AF. I've limited experience (tried my friend's 70-200L IS) applying this technique but I think it is useful. Even Micheal from Luminous Landscape complaint about it (rather lack of) when he reviewed 7D with the older lenses last year. I wonder if HSM (S) can do the same.. :think:
6) Most obvious different.. with SSM (KM).. assuming it should help in AF speed and obviously it will be more quiet and smoother when AF.
In conclusion, is above extra features and differences able to help you to justify the different in pricing? ($1500 like Sulhan said?) Do you need them? I know forbytes always complain about AF noise in quiet place (valid reason!) :bsmilie:
Or...
Just simply say, "I support Minolta!" :thumbsup:
Just my 0.02.