Sun Tzu. Veteran units rule. There are a couple periods where you can produce Veteran units that can sweep all in their path (Crusaders, Ironclads). The Ironclads may need to win in combat to become veteran--so pick off an onshore caravan, elephant, or catapult.
These mismatch periods offer you an opportunity to completely eliminate opponents and significantly "change the map" over the course of very few turns. I use my veteran crusaders to capture the cities where other wonders have been built.
Mike's is really helpful too, especially on Deity. When it kicks in your production goes way up--adding 50 or more shileds per turn easily. I call it the "peace dividend" of Civ. Cities go from scraggly to healthy little growth engines.
This is a close call, as both are valuable. However, as noted, one of the benes of Mike's is stable cities after you conquer them. Sun Tzu's is the engine that allows me to conquer them in the first place. With Sun Tzu, my newly conquered cities don't need as many defenders, as the conquering unit (a vet) is usually enough to hold it.
Also--I really HATE trying to fight a civ that has Sun Tzu's and all vet units. I have to totally switch over and build zillions of diplomats, and regear my war economy for considerably more trade and cash.