Doom 3 vs. Source: Comparing Engines
Tom V. writes "DevMaster.net has an article that outlines some of the technical differences between Half-life 2's Source and Doom 3 engines from various game development aspects such as graphics, A.I., physics, networking, etc. According to the author, the winner is the Source engine based on its 'completeness' as a game development package. However, in terms of graphics, the clear winner is Doom 3."
Valve explained this once. The lake is the source, the filled glass is the game, and the faucet is the valve.