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There's only about a stop in max aperture, and the extender is only going to make it worse (granted you'll have massive reach). I use Nikon, so I'm not caught up on Canon lenses, but barring any HUGE differences in lens quality (which I'm assuming there isn't since they're both L lenses) I bet that you wouldn't notice any difference in IQ between the two.
I'd go for the zoom for the versatility. Also make sure you have a tripod or at least a monopod....
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