I am looking at my next purchase of a lens and I think I have narrowed it down to either of these two.

I would use this lens for outside portraits of people and maybe some wildlife shots/zoo's etc. I would also use inside for portrait shots as well.

What is the weight difference between the two? I know the F4 is light and I am thinking it might suit me, with IS for low light, but I am drawn to the 2.8. Both lenses are around the same price. Which would be better for low light shots? I have concluded the 2.8 IS is just way to expensive for me to buy - was my original choice.

I am really interested in your thoughts and do realise it is ultimately my call, but would appreciate some for/against thoughts.