I have used the 70-200mm f/4 L IS USM- and I own a 7D.
However for my purposes, the 70-300mm f/4-5.6 L IS USM won from a perspective of having an extra 100mm reach, and being almost as 'fast' as the f/4 for the same mm (it stays f/4 for quite a while, and then f/4.5, etc). So I bought the 70-300mm L soon after it was released.
Most reviews will show that both lenses are very sharp, even wide open. My 70-300mm L is still very sharp at 300mm f/5.6 (the setting I use most) - but it is a bit sharper at 70mm (if I look closely). But at 300mm f/5.6 it still knocks the socks of most lenses. Though it needs to be taken into account there is sample to sample lens variation in IQ.
The 7D is definitely more demanding on lenses than eg an 8MP or 12MP equivalent APS-C. I have a number of lenses, and eg the 15-85mm really outshone my (now sold) 28-135mm on the 7D. But on the 350D, while the difference between these 2 lenses was noticeable, the difference was not AS noticeable as on the 7D.
I can highly recommend the 70-200mm f/4 L IS USM, but recommend the 70-300mm L if you can afford the extra bit of money. The 70-300mm L is very hand-holdable (it's 4-stop IS is very good, and its USM focus very fast & accurate). Though built like a tank, it's not too heavy to transport (or have in a bag) all day. I like it's 'stumpy' design and that the zoom ring is at the end of the lens (that's what I prefer!).