My understanding (based, if I recall correctly, on R' Moshe Feinstein) is that when I pray in a congregation that uses a different version of the prayers (nusach) from the one I usually use, I should use the congregation's nusach whenever it's readily apparent to people around me which nusach I'm using (e.g. parts which are said aloud), but I may use my own nusach when I can do so unobtrusively (e.g. the silent Amida).
What if I'm leading the prayers? Clearly, for nearly everything, I'd use the congregation's nusach, but what about for the silent Amida, which is, of course, purely silent and presumably private? Is the rule different for prayers that include a repetition of the Amida (morning and afternoon) and for prayers that don't (night)?