mephedrone is much more toxic and addicting that mdma.Better to be floored that flooded with cardio toxins
More addicting? Definitely.
MDMA is very difficult to get truly addicted to. You just can't pop MDMA all the time, because the effects will rapidly diminish and the sideffects will get worse and worse. You just can't reach the same level of addiction as you can with meph.
Meph is redose-friendly (fiendly?), and lots of people were doing it daily, and regularly going on binges, multiple times a week, etcetera during the UK meph craze. People describe binging until they started to get psychosis-like effects, then crashing, recovering for a day or two then going on another binge, etc, for months. Plus, as the experience is less overwhelming than MDMA, it's more natural to redose, and easier to hide the effects of from others.
Meph is has far more addictive potential than MDMA (an addiction to which is effectively self-terminating because it will stop effecting you).
As for toxicity, neither of them are good for you.
MDMA is toxic to serotonergic neurons (though, clearly, not overwhelmingly so, as huge numbers of people have used it, some quite excessively, and the majority of them are none the worse for wear), though it's relatively benign physically.
Meph is less neurotoxic, but users make up for that by using way the hell more. Meph tolerance seems to carry over to MDMA (or so some have said). It's also much harder on the body, with particularly strong peripheral sideffects - vasoconstriction (leading to coldness in the extremeties, or in extreme cases, legs turning purple/blue), rapid heart rate, high blood pressure, and the resultant damage to the cardiovascular system.
I'd say that with equal intensity of use (ie, 1 dose every 2-4 weeks), meph is probably safer, but meph lends itself to a MUCH higher intensity of use.