This is a question quite a few people have had when upgrading along the telephoto line. Naturally it would be good to review your needs and wants before jumping in to buy a lens. Hell, even I should start doing the same. Over time it becomes apparent that it is annoying (not to mention the possible loss of money) to continually buy lenses, find that it's not right and have to change for something else.
As far as I know, there is actually a significant difference in quality between the new 70-300mm IS and the old 75-300mm III. In fact, one source even stated that the 70-300mm IS may be a hidden L lens because of the quality of images it produces. Do be aware though that this lens produces softer images when used in the vertical format because of a glitch. This is something Canon will fix for free, so check it out with the Canon Service Center down at Harbourfront to see if yours needs fixing.
Now let's talk about the bigger stuff. Both the 70-200mm f/2.8 IS and the 100-400mm are mighty fine lenses with their fair share of followers. The 100-400mm is more geared towards wildlife and maybe outdoor sports in good light, compared to the 70-200mm which many will use for a myriad of purposes...maybe events, weddings and wildlife, provided it is close enough. It will also do indoor sports because of its bright f/2.8 aperture.
With that out of the way, both lenses feature tremendously different focal length ranges. I've not used the 70-200mm f/2.8 before, but have tried the f/4 version and previously had the 100-400mm. I sold the f/4 in the end because the range wasn't right for me. I'm primarily a landscape and wildlife fanatic and thus need something that zooms very far. That said, I ultimately sold the 100-400mm as well because of a lack of funds and a burning desire to upgrade from the kit lens. I still miss it today.
In short, think about what range you'll need and then choose from there. Both the 70-200mm f/2.8 and 100-400mm have excellent image quality, but it's a matter of how long you need your lens to be. It is not recommended to attach a 2X converter to your 70-200mm lens. Yes, it will give you a 140-400mm f/5.6, comparable to the 100-400mm, but the image quality will not cut it until you stop down to f/8 or f/11, or at least that's what the reviews show. Try looking here for more info on these lenses:
www.fredmiranda.com
www.photozone.de
www.photodo.com
www.luminous-landscape.com
www.the-digital-picture.com