The 2.8 is definately heavier (and the IS does add some weight). You get used to it though (and your neck gets a lot bigger :P)


Another pro for the 2.8 - if you decide you want a little more reach later on, you can always grab the 1.4x extender and then you're looking at an F/4 lens with a bit more reach if F/4 will do in certain situations. If you get the f/4 now, you'll have a 5.6 lens which is really beyond the land of usefullness for a lot of sports.