Hi Mark,
IMO, an f/4-5.6 lens doesn't deliver enough OOF blur for portraits, especially on an APS -C sensor. You'll only get decent OOF blur if you can put a *lot* of separation between your subject and the background. One option for portraits would be an 85mm f/1.8 - it's an excellent portrait lens on a crop body.
I agree that IS is a real benefit at the long end - unless you're shooting sports, you'll often be able to use much lower ISO with IS.
Personally, I like the combination of a slower zoom for general use with a fast prime for portraits and moving subjects in low light.





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