The difference is this: Amphetamine effectively inhibits the activity of the VMAT-2 transporter, so it packages up less dopamine. Methylphenidate (the chemical name for Ritalin), on the other hand, enhances the activity of the transporter. And this difference is actually what makes amphetamine neurotoxic and methylphenidate comparatively benign. The thing is, dopamine is a very reactive molecule when it isn't packaged by VMAT-2, and when it autooxidizes in the presynaptic cytosol, it can actually damage the presynaptic terminal. Amphetamine accelerates this, and causes presynaptic terminal damage. Meanwhile, methylphenidate prevents it from happening.