The difference is the Interaction in the brain differences:
Meth, once it enters the blood stream, crosses thru the blood-brain barrier, stimulates the dopamine receptors to pour out dopamine and also blocks the recycle process to recover the dopamine. It takes the body 12+ hours to break down the meth to cause this activity to stop. During the time meth is actively changing brain chemistry, the body has a 3rd process which destroys excess dopamine, which is NOT blocked by meth, so the body goes about destroying a lot of dopamine for hours at a time.
Cocaine, once it enters the blood stream, crosses thru the blood-brain barrier, stimulates dopamine receptors to pour our dopamine. It does NOT block the recycle process. The body takes only a few minutes to break down cocaine, which is why the "high" from cocaine only lasts a few minutes.
Both cause over stimulation of dopamine receptors, causing some damage there.
Meth is worse than cocaine, simply from a brain chemistry prospective, because it blocks the natural recycle process and stays in the brain for so long before the body can break it down. It disrupts brain chemistry longer, and more harshly.
Meth is extremely addictive - take a look at the pictures of people who are on it - its just all wrong.