The 75-300 is known to be a bit of a dog.
Ronny Nilsen ("ronnynil" here) performed an unscientific test of the differences between his old 75-300 and his new 70-200 f/2.8L IS (which supposedly is slightly inferior to the non-IS), and was kind enough to leave
his test on the web. While he doesn't provide the full-size images as you request, I think the test still shows something of interest.
Brief explanation: the image taken with the 70-200 also included the EF Extender 1.4 II, which in my experience results in noticeable (but not horrible) image degradation.
Even so, pay attention to the differences, not only in sharpness and resolution, but also in contrast -- even in the reduced size image.
If you can spare the money, I'd say go for the 70-200, but perhaps you should also consider the 70-300 lenses with IS.