It would appear that MDMA does indeed have subtle long-term effects on complex memory and executive functions that are independent of cannabis and may remain with abstention. This is consistent with evidence of disruption of the serotonin system in animal and human studies.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1525102/

Ecstasy users were unimpaired in simple tests of attention (alertness). However, they performed worse than one or both control groups in the more complex tests of attention, in memory and learning tasks, and in tasks reflecting aspects of general intelligence. Heavier ecstasy and heavier cannabis use were associated with poorer performance in the group of ecstasy users. By contrast, the cannabis users did not differ significantly in their performance from the non-users.

http://jnnp.bmj.com/content/68/6/719.short

The manifestations of this neurotoxicity, in terms of altered cerebral function and behavioral change, range from neuroendocrine impairments to deficits in verbal memory and reasoning,short-term memory and semantic recognition, and visual memory. More general indices of intelligence are also adversely affected, but reports of serious long-term psychiatric disorders are still rare, with the possibility that previous exposure to MDMA merely accentuates preexisting negative personality features. One particularly worrying feature that has emerged is that chronic psychosis, when manifest in MDMA users, reportedly responds poorly to therapy

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071023/

MDMA is a more harmful substance than cannabis. It is well documented. Accordingly, you should take MDMA with care and in moderation, or else you may find yourself with some cognitive damage. Hell, I don't even think that it should be illegal, but at the same time it is not a substance to be taken too lightly. It is a powerful psychoactive compound which can bite you in the ass if not treated with respect.