We don't do "drug compariosn threads" but I'm a Nice Guy so I'll answer you.
Methamphetamine is a powerful monoamine releasign agent that is balanced towards norepinephrine and dopamine - it has a long half life and sticks around in your body for a long time.
Mephedrone also releases significant amounts of serotonin, like MDMA - and as it is a cathinone and not an amphrtamine, it's metabolized faster and has a shorter duration of action.
So in summary methamphetamine is more of a "plain stimulant" and mephedrone is more of a "party drug". From what I've gathered, anyway.