That's because neither format supports bones ( technically latter versions of the .x format support bones but Blitz can't open these, so it's a moot point. )
You must use b3d to have proper skeletal animation. You may choose to use MD2 if b3d is not an option, and you will get the animation, but you will not have access to the bones for anything else you might want. B3d is generally the best choice.
You must use b3d to have proper skeletal animation. You may choose to use MD2 if b3d is not an option, and you will get the animation, but you will not have access to the bones for anything else you might want. B3d is generally the best choice.