Well, in a smoothie you can use either because you will be blending them.  But if using flax in other non-blended foods I suggest using ground flax meal.  Your body is unable to digest the flax seeds, they simply pass right through your digestive system and you don’t receive any of the nutritional benefits.  However, if you happen to buy flax seeds you can grind them yourself in a spice grinder or blender.