I'm just a seventeen year old feeling haughty because I'm doing second year physics but...

Yes a good bike messenger is fast, but a bike messenger would be doing alot of stopping and starting, especially in heavy traffic. Therefore a 26" wheel would be better in this situation because it will have a lower rotational inertia and thus accelerate and deaccelerate quicker. As well as this, It'd be less likely to go out of true.

That having being said, 29" mountainbikes are really cool, and I want one. Virtually nonexistant here though. I envy you americans and your lower prices and weird eccentric bikes.