The f/4 IS version is significantly smaller and lighter, and compared to the Mk I version of the f/2.8 IS, the f/4 IShas better image stabilization and better image quality - but it's f/4. If you need the extra stop, you need it. The Mk II version of the f/2.8 IS has equivalent IS and IQ to the f/4 IS, but of course the price differential is much greater. So, the question is, do you need that extra stop either for light gathering or for OOF blur? If so, you either sacrifice some stabilization capability and IQ (and some &pound for the f/2.8 Mk I, or you sacrifice your pocketbook and much more £££ for the f/2.8 Mk II.