The bottom line is budget. I have the IS
version, having previously had the standard lens. There is no doubt that the IS
lens provides a lot more flexibility - for example, being able to keep the aperture relatively small (to help with the photo quality) without losing the photo to blur. Also, contrail shots are possible with this lens that I don't think would be with the standard one - again because you do need to watch your f-number to maximise quality.
So the IS
really does make a difference, but there is also no doubt that the lens is somewhat soft at the long end of the zoom. You can read more information in this thread
All I can say is I am glad I made the upgrade. But I still am a bit envious of those guys with the 'L' lenses - they do make a difference to quality. Though with good editing technique the IS
lens is more than acceptable.
Hope this helps.