gugu wrote:I actually wanted the
L to win.
Are there any reasons I should change my mind?
.
I feel strongly about the 70-300 when trying to think objectively.
I feel that YOU should win, not the lens
and by winning, I mean picking the lens that suits your needs. I went through this exact decision too. I've shot the 70-200 f/4L (on a 5D too! beautiful.. both the lens and camera) and the 70-300 IS USM. Both are similar in price.
I actually like white and red..
but I felt that I should be buying something that suits my requirements. My 17-85 introduced me to the magic of IS, I've been able to pull off shots that would have simply been impossible. IS will not replace shutter speed, but I always for some reason find myself in situations where I can get enough shutter speed to stop action - but not stop handshake. This is where IS will be truly worthwhile. There have been rarer situations where I shoot in good enough light that I can simply turn off IS and use all the shutter speed I want (1/1000-1/1250 at f/8!), but those are few and far between.
IS will preserve the lens' useability in more situations. For everyday people, say, like you'd find on the streets and at events (vs sports, action) - you don't need that high a shutter speed to 'freeze' them. You may also find yourself shooting in lower light (indoors is a killer).
For shutter speed, try cranking up the ISO a little bit. f/4 constant and f/4-5.6 variable aperture aren't that much difference especially since bumping the ISO up one speed gives you a whole extra stop. If we were discussing the f/2.8L vs the 70-300.. then of course, there is absolutely no contest 
Image quality is outstanding. I took a picture of the crowd at an event and I could even read the tiny little name tags they handed out! Reach is good too, some days I find I need ALL the reach I can get, and that means 300mm on a 1.6X camera (480mm equiv FOV). I'd rate it on par with the 70-200 f/4L I handled.
Build quality of the 70-300 IS USM is not on par with the L, unfortunately. I get the 'feel' that the L can handle anything I throw at it, whereas I have to baby the 70-300 (which I generally do with my equipment anyway..mmmmm...precioussssss
) The zoom tends to creep as well, when pointed down. Judicious use of the zoom lock in storage is advised. I may be lucky - I can't deny the results of fstopjojo's excellent tests about this lens' flaws in portrait mode, but I have not experienced them on my copy. I hope mine continues to hold up as it gets used!
Now if Canon added IS to the 70-200 f/4L at the same price point, I wouldn't even think about the 70-300 IS USM! Guess they'll keep me waiting quite a while for this, though.
Good luck with your decision. I hope it works out for you!