Bit of advice please, apart from the built in IS is there any difference between the IS & non IS versions of this lens.

Thanks

About a doubling in price and the ability to work in lower light levels, (3 stops iirc).

I have the 2.8 IS and not sure I would opt for the IS option if I bought again, I have captured images that I would have missed without IS but not sure I can really justify the cost difference.

I don't have the 70-200 in either form but I do have the 100-400 IS and to be honest I wouldn't want the lens without IS. Maybe the longer focal length needs it more but I find it so reassuring when I half press the shutter and see the image lock solid in the view finder. IS is FAB! (IMHO :D)