I used to shoot weddings back in the day, with a Mamiya Pro-TL and a bunch of prime lenses. At the time that I felt that I had to go digital I would have happily gone to a digital back for the Mamiya, had it been possible and affordable, and would have continued to use those Mamiya primes. But I couldn't, so I bought a Canon 10D and a lens system that included primes and zooms.
Within a short period of time I realized that the zooms gave me opportunities to get shots that I would have missed with primes. The primes might be technically sharper but the zooms - at least the Canon L zooms that I have - are acceptably sharp. (That sounds like a back handed compliment at best but what I mean is that I can look at an 11X14 print from one of these lenses and be satisfied with it, while recognising - at 100% on my monitor - that the zoom image is not as sharp as say what my 100mm macro may give me.) I just came to the conclusion that a shot that I was able to take with acceptable sharpness was better than having superb sharpness in a shot that I missed.
So unless speed or depth of field creativity is the deal breaker I'd suggest the zoom. By the way my 70-200 f2.8L (non IS) is remarkably sharp - in my opinion prime sharp - and the Mark II should be even better.
"There's never time to do it right. But there's always time to do it over."
Canon 5D, 50D; 16-35 f2.8L, 24-105 f4L IS, 50 f1.4, 100 f2.8 Macro, 70-200 f2.8L, 300mm f2.8L IS.