I don't get why you think heavies see so much more use than other units. Does Gellius have incriminating pictures of you or something? Anyway, the biggest thing that gives me pause about recommending Sophie+Matsu over Deine is that you really want to skill up Sophie to get serious use out of her and new players obviously don't have a fleet of Niels and rainbow fairies to throw in her direction. Otherwise Sophie is just generically good and fits into compositions pretty easily--worst case scenario she provides a dab of sustain and ganks so hard that it turns into wave clear. :P