I've used both, although on different bodies. I have used the 70-200mm f/4.0 L on a 20D and 1Ds and the Canon 70-300mm on a 350D.
What I can say, the 300mm is much nicer then the Sigma equivalent. I went to France at the start of year with Phil from Neobuggy to cover a race and he had a 300mm Sigma lens, and it was noticeably slower then the Canon one. Not really important, but noisy too (compared to Canon's silent USM). Not a bad lens, but I ended up borrowing his 200mm L, much nicer.
The L lens is nicer, I think there is no doubt there. Less distortion with the photo, not that the Canon 70-300 is bad, I got nice results with both, but when you look at the photo closely there is a difference. Another thing, the 70-300 is actually f/4.0-f/5.6 if I recall, this means it will be slower at the full zoom range whereas the 70-200 is constant f/4.0.
Another consideration (sorry I know I can type forever) if you go for 40D or 450D, they are both APS-C sized sensors. This means you get a 1.6X cropping factor compared with a full frame, which makes the 200mm effectively 320mm compared to people with full frame cams. So have that in mind when reading reviews if the person is using a full frame.
I always find 70-200mm plenty long enough on an APS-C camera, and it's even a little lighter then the 70-300mm. So my vote would be on the top notch L glass, there is a reason it's so popular I think
