I would say triangles, which most cards are better optimized for these days. However, I think quads are just broken into triangles these days aswell... I personally use triangles.

Alex Broadwin
A-Tronic Software & Design
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"if you fail in life, you were destined to fail. If you suceed in life, call me."
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If its a simple cube then I don''t think quads would impact the frame rate very much. But if you start rendering bigger meshes (like terrain for instance) you''ll probably want to use triangle strips.