I tried both, and purchased the L lens. One thing to note, is that the IS USM lens is 5.6 at 300mm, and something between at 200mm (don't recall exactly). The L is f4 at 200, always. As you probably have discovered, both lenses are quite close in price.

True, the IS USM is 300mm, and it is IS. IS is nice when trying to shoot at 300mm and handheld, especially when you do not have a tripod and especially when your shutter speed is near 1/300 or lower. So, if you expect to find yourself in these situations often, then perhaps you should lean in this direction.

However, the L lens is brighter, focuses impossibly fast, has manual focus override, and it zooms internally, so it stays the same size always, never extending. But, even with all that, the difference in sharpness and clarity are simply amazing, and still, after thousands of shots and 3 years, bring a smile to my face, and causes giddy pixel peeping every time.

The 70-200 f4 L lens, is, in my opinion, the best bargain among the L lineup, and just maybe the entire Canon lens list. I can highly recommend this lens, and unless you absolutely must have 300mm, you should purchase this lens.

To see these differences in clarity and sharpness, be sure to look at these comparisons that are available on the-digital-picture.com. Compare the images at 70 and at 200mm.

On the other hand, there is a 70-300 f4 L lens available, but its just a tad more expensive (!)