Your typical difference is simply that traps and futanari are reversing different roles. The most common train of thought here is that sticking a donger on a girl will hit her with that constant fuck-lust of a man, maybe even worse, and it draws her out of that girlish sense of reservation. It makes her proactive and dominant. The common thoughts on traps are the reverse, that the effeminate figure and the way they're treated triggers a submissive reaction, that it awakens the same desire that went dormant in the futa.
but it's important that boundaries are observed. A boy flat-out becoming a girl or vice versa is a different thing (along similar lines, sure, but ultimately these are not about the loss of one but the awakening of the other, gender bending is all about the loss) and, to be quite frank, it's rare for a futa or a trap to gain a genuine sense of the opposite side - but I fault the typical writing for not exploring it all that far. What you really end up with in the typical trap scenario is the state of effeminateness, swooshy, what's often disparagingly called fairy or sissy behavior. And with futas it's a tomboyishness, disparagingly called dykey, boyish not as genuinely masculine but in such a way that would turn off a guy looking for that softer femininity that (again, frankly) he feels he can sink his hands into and take, throw around, control.
But they're far more versatile concepts than simply turning boys into sissy-things and girls into horny hump-crazy perverts. The best traps have an interesting mix of more genuine girlishness that doesn't diminish or belittle their boyishness and futa that really strive to answer the question of what a girl might do with a dick are so much more fun. At their full potential they're probably two of the most versatile archetypes.