One thing I have read (I know you cant believe everything you read on the internet but bear with me) is that the 70-300 is quite a bit sharper than the 75-300.
I've heard that too, from the guy who sold me the 70-300 IS, which I quite enjoy.
What it came down to in my mind is I think the IS might be more useful for me than the L optics.
I came to the same conclusion and purchased the 70-300 IS. The 70-200 is a great lens as well, but I felt the IS would make a difference for me for wildlife shooting. I was more or less talked into that on this forum, and I'd say it turned out nicely.
My reasoning was that I'd benefit from the longer max focal length of the 70-300, and that IS would be really beneficial at that length. The 70-200 may be optically superior. Having said that, I'd end up needing to use a teleconverter (which isn't free) to get the zoom I wanted, and in that case, I might be suffering without the IS. I think it's hard to keep the camera and lens steady at 300 (at least on a consistent basis) without IS. Other disagree, saying you're better off improving your technique. That's not untrue, but there's only so much I can do to steady a camera when shooting wildlife that tends to move quickly. If you want to do a different type of shooting with it, maybe that's not important to you though.
I think if it comes down to paying full price (~$600) I'm going to have to try both of them out. I was thinking about going to a shop and taking some shots with both and then comparing the results at home.
If you can try them both out, DO IT!! There is no substitute for you holding them, shooting with them, and looking at your photos with each. What works for one person might not work for you, so a hands-on is always prefferable if you have that option.