depends on the game engine. Some game engines are happier with nurbs others (most) are happy with polygons. Main reason being that most systems cannot push more than several thousand polygons at 30fps. and you have to budget over the entire scene, not just the character. Quake uses nurbs to extract polygon geometry. e.g. over a curve it will build polygons that it can use. Poly modeling is most common in games simply because of the explicit control that an author can have over the geometry of his object. With nurbs the author cant be sure if the engine will transform curve geometry into triangles how he expects it to.
-alex okita
Bungie Software
http://oni.bungie.com/