Mischief, please understand that Dave is hardly an unbiased person on this topic. For example,
see his posts over on this thread.
To get an unbiased answer to your question, we would first have to find out what you mean by "the most powerful." For example, are you talking about research? What? Bottom line, despite what Dave says, there is no easy answer to your question.
Generally, SE will give you better early game research. CE will be better starting around 1000AD or so (especially after you get the bonuses from Paper etc.) Of course that varies from game to game. Some other comments: SE can sometimes give you better early-game production because it allows faster regrowth from whipping. Late game, if you are going for Domination or Conquest, SE can outperform CE, because the cities you conquer often have farms etc instead of matured Towns. Regardless, CE almost always is a late-game research powerhouse (you may not care about that, is my point, if your game plan is to get to Cossacks and then to conquer the world, for example). If you are going for a Space victory, then CE is almost certainly your best option.
Choice of leader, financial or philosophical or other, and whether or not you have stone/industrious (or some GE scheme) to easily get Pyramids, affects things greatly. In addition, terrain of the map you end up with will affect the situation. Some maps simply will do better with one system or the other, or with a hybrid.
Regardless of the system you choose to use, skill of the player will be important. Optitmal use of things such as a GP farm, religion, and your tech research choices will have a huge effect.
Wodan