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I had the 70-200 f/4 L on my 7D. It was fantastic at the shorter end, but the long end didn't reach far enough for most of my uses. I debated for a long time about replacing it.
I made up a spreadsheet comparing the parameters of all the tele lenses that could fit the bill, and it came down to either the 100-400 or the 70-300 L. The 70-300 won on every count except the max reach.
I ended up going for the 70-300 L. So far I haven't really used it in anger, but I'm about to head off to South America for a month.
I'd suggest you try to get hold of a couple of lenses and try them on your 7D. Either of the lenses you are considering, or the 70-300 L, are quite heavy when combined with the 7D. They are much heavier than the 70-200 f/4 L IS that I had before.
The 100-400 is no slouch (though I've never used one myself), but the 70-200 f/2.8 L IS, and the 70-300 L IS are sharper, have better IS, have better AF, and are water resistant. These are all considerations that help compensate for that missing reach.
I figured that for my use, 70-300 was a very good focal range. The 400 f/5.6 prime is supposedly up for replacement soon, and it's replacement will have IS as well as more modern AF. When that one comes out it will definitely be on my wanted list for when I need the extra length without the versatility of a zoom.
Cheers,
Jason.
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