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Max, if you are hand-holding any IS helps, 2 or 3 stops in the case of the 100-400L. If you're gonna shoot with a tripod IS doesn't matter, so you just need to decide if the extra range, one lens solution will cover your needs. If the reach of 400 is all you need the the prime might be your best bet, even if just a little AF will be faster and the glass sharper; I'd suggest you shoot from a mono or tripod tho, unless your long lens skills are rock steady...
70-200 f4L's are fantastic lenses, I have the IS version, it's my most used zoom!