This is probably the fourth time I've seen this question appear, so I'll try to set the matter to rest. Which is better, Leo's or Sun Tzu's? The answer, like so many other things, is "it depends."
Are you playing on a pangea map? Sun Tzu's is extremely powerful on pangea maps, almost useless on archipelago maps. How large is the map size you're playing on? On a tiny map, Sun Tzu's will only put barracks in a handful of cities. On a huge map, it can potentially give you free barracks in hundreds of cities, saving thousands and thousands of shields. What is your victory goal? For conquest or domination, Sun Tzu's is far more useful than if you are going for a different victory type.
How many units are you planning on upgrading? If you have specifically tailored your strategy around upgrading units and attacking, Leo's can potentially be the strongest wonder in the entire game. This requires much planning and foresight to set up, of course. What units are you planning on using? If you're using horsemen or knights and planning on upgrading them later, Leo's is very powerful. If you're using longbows, MDI, or cavalry, then the wonder is all but useless to you. Keep in mind that you can also only upgrade units at cities which have barracks.
Think about your situation and what your goals are, and you should be able to figure out which one works better for your game. Leo's works best with some very advanced upgrade strategies, so for a novice player Sun Tzu's is probably the better choice. It's all situational though, depending on the map and what victory type you are trying to achieve. Anyone who tries to assert that one wonder is unilaterally better than the other doesn't know what they are talking about. Use your head, think about these factors, and determine which ones works best for your game.
