ATM it's tough.
Mara plays like a Muti rogue....low skill cap (For melee)
Assassin plays like a DK.....an 8 year old girl could play the class, (My class)
Mara will ALWAYS be a lower skill cap class,
Assassin might go from 1 of the easyest to one of the HARDEST if the DPS spec gets any kind of buff.
So in short, I really don't know.
But you want a Melee class that plays as easy as a caster....you can't go wrong with either pick.