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I own the 150-600 and could try the 60-600 at an exhibition 2 weeks ago. It feels more heavy, even it is only 200g more weight. The IS seems to be better (that was also stated by the sales guy from Sigma). I was shooting handheld through the exhibition room with 600mm and 1/60 with sharp results. That was quite impressive. But I could not really test the AF.
But actually there is a price difference of around 1000€ (in Germany). If you look for a tele lens, the 150-600 delivers very sharp results for a great price.
I tried to upload some pictures, but I always got the answer "too large" . The last try was below 1MB per file... I have to check the maximum size and will com e back...
Ihm happy with the 150-600 and don't want to change: The 60-600 is more heavy then the 150-600 and the 150-600 got the new OIS via FW update (which was really a step up). The higher zoom range is (for me!) less important than the weight.
Not sure if this helps, but there are comparisons out there on the web. Summary - incredibly, the 60-600 is sharper at most focal lengths. Has better stabilization. And faster AF due to faster motors.
Seems like a perfect "safari" lens, or in situations where you don't have to carry it far and can keep it mounted on something. I suspect that faster AF will be super helpful when coupled with the new AF capabilities of the S5II.