'transgenderism' implies that it's an 'ism', which since 'ism' means a belief it implies that being trans is just sort of a 'belief' as opposed to a firm, solid point in someone's entire identity. plus the word also was used in the mental field to 'diagnose' being trans, and since then it has been shown that it is not any kind of mental disorder! 

plus, saying 'trans' is a lot easier to type out, i think