I think the true rivalry between the now 3 major game engines will occur once 3rd-party developers start licensing these engines for their own games.
For many years now it's been either the more traditional Quake or the cross-genre-friendly Unreal. With the Source engine's new direction towards "reactive" and "emotional" gameplay, I think we will increasingly see FPS games being described as belonging to an additional 3rd side. Counter Strike 2 (Just at the top of my head, sorry.) may have perfect ballistic engine physics and destructable environments, but it may not match Quake 4's real-time photorealistic graphics. Neither of these will have the emergent RPG storyline of Deux Ex 3.
Then again, considering the number of successful games in the recent past (Mafia) and potential future (STALKER) that have used or will use their own engines, any rivalry between the 3 could be deemed insignificant. The Source engine may also have some as of yet undisclosed RPG capability, making it compete more directly with Unreal.
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