In a player vs. AI game, I combine the two.

For instance, as England, I rush to unite the islands, then settle down to create a steady economy and a military infastructure. When it comes to attacking the continent, multiple large stacks, each containing a core of elite units, are more effecient than a gajillion massive stacks of peasants. If you ever get to the point where you can't continue without excommunication and rebellion, you can always go back into turtle mode and still defend yourself well until you get your initative back.