I have tried both many times, and must say i much prefer mmcat. the high is far more euphoric, far purer, and the comedown is fully minimal. the description below is of my experiences of heavy mdmc (methylone) use and mmcat (mephedrone) use :-
personally, methylone comedowns leave me feeling dissociated and dependant, unable to concentrate or do work, detached from the real world, and self-destructive (even masochistic). i have found it nigh on impossible to sleep as mdmc is so incredibly moreish that i get restless and frustrated when it has run out. not a fan personally, however when its around, in my old friend group... theres alot of it.
the comedown can be put off for days at a time as long as you keep taking it, however the initial euphoria is hard to regain until the next separate session (not impossible, just difficult, especially if youre administering the powder nasally (snorting))
HOWEVER
with heavy use of methylone, especially after a session of at least 12 hours (a session on either drugs is timed only by how much you have. 'once you pop you just cant stop (until its all gone)' i would advise smoking weed, maybe one average joint per half hour. the THC mixed with the side-effects of a methylone comedown provide for some extremely bizarre trips, mostly mental/emotional trips rather than visual, however if youre into viewing the world from a completely different viewpoint... you need to experience the daze of being stoned and on a methylone comedown.
sorry for the essay, and i realise i havent even written about mmcat, however that ispurely because, in my experience, there hasnt been a single remarkable comedown from mephedrone, just soome reallllly deep 12-hour plus sleeps! (the closest is some waking dreams/hallucinations while trying to get to sleep) and the high from mmcat is honestly what i imagine the supposed euphoria right before you die feels like. this stuff should be called angel dust imo