Let's look at these two a bit more closely.
Ranma Saotome, star of Rumiko Takahashi's Ranma 1/2, has his female transformation forced upon him from the outside. It is literally a curse that he has to bear. Even the trigger for the change, water temperature, is external to him.
Thus, the change in Ranma's appearance does not change the way he thinks about himself. He may mimic female behavior patterns when it's convenient for him, but he never internalizes them. This is even more notable in Japanese, where he does not use feminine language in female form unless he's specifically pretending to be a girl.
Contrariwise, the transformation of Futaba Shimeru, star of Hiroshi Aro's Futaba-Kun Change!, is internal in nature. It's inherent in his/her genes, and despite his initial mental discomfort, is natural to his/her species. It's shown that Futaba's brain chemistry and way of thinking change slightly when in female form. Only the surprise of the transformation and lack of socialization really cause her to have problems with being female. His/her trigger is likewise internal, based on emotional state. And as the series progresses, Futaba slips into feminine speech when a girl without even thinking about it.
It's also notable that Futaba-Kun Change! focuses more on the body function changes between the genders. Ranma 1/2 treats female Ranma as more or less a meat disguise for Ranma, while Futaba pees and bleeds "like a real girl."
Both approaches have interesting things to say about gender roles and identity; I hope you'll find them worth your time.