Even though I am like many and crave low light performance I did the opposite to what most do. I wanted the new f2.8 IS II - until I saw how big and heavy it was. I wanted it mainly for travel, so size and weight was more a consideration than aperture and price. In the end I opted for the f4 IS version. It has lighting fast and deadly accurate auto focus on my 5D2. It is light enough that I can lug it around on my 5D2 with grip and Black Rapid strap without fatigue issues. The IQ is faultless and it works well with my 1.4 teleconverter - it actually reads f5 instead of f4 when it is wide open, not f5.6 like you would expect. I have other lenses to use if I need a wider aperture with a shallower depth of field so it fits my needs well. But that is not to say it will fit your needs. My suggestion is to go into a store and actually try both the f2.8 and f4 versions out, that will give you a much better idea on which way to jump.