We love our pups!! Misu, RIP 28 Nov 2010. Tikka, RIP 11 Aug 2011.
If a map tweaks things such that, for example, all your builds must be contiguous, it would make a difference since you couldn't urbanize a city away from your network and start building from it. But for the basic rules, it shouldn't make a difference. However, enforcing urb before build might be good for beginners so that they don't forget they could urbanize away someones track stub (from a town) and start building from the new city. So if you have to choose one or the other, I'd choose "urb first then build track" over "urb before/during/after track builds" (plus, it's simpler to say/explain/write!).
Some maps (eg London) require that towns be built before they can be urbanised. Requiring the urbanisation first would preclude building and urbanising a town as part of the same track build. Similarly, Sun's urbanisation moves cities to other (uncoloured) city locations which might have been built during that turn by the player. Other maps require that all of a players track is contiguous (West US?).