From my experience in Civ3 and Civ4, and from what I've heard about Civ5, Firaxis expansions are few and actually excellent. You don't even think about playing Civ4 without its expansions.
On the contrary, Paradox shower its games with expansions and mandatory patches, many of them being trade-off between "I really like this part, but I hate this one" and sometimes ruining the game in the process (Cherry, I'm looking at you, I've rarely felt such an undying hatred and contempt for a patch).
When someone ask about a Civ game, it's a no-brainer to get the latest version with everything.
When someone ask about a Paradox game, usually the debate is about "which DLC should I get and which ones should I avoid ?".