Tweening or frame by frame

Hi is it less work ofr flash to do if i use frame by fram animation instead of a motion tween. Why I ask is that I have got a simple animation in my flash that move an image from one side of the screen up and down and round etc… to the other. This happens via a setInterval call which is cleared with a clear interval… however I realise that after a while it begins to get choppy each time set interval calls it to play… my frame rate is 20 fps… which is better?

Thanks…