It's contestable both ways. Some would argue that they're different, but I would say most people outside Nepal only use "Nepalese" and it sounds correct to them. For most Nepalis (I could say Nepalese here!), we grew up calling ourselves 'Nepali' so we would rather use this word. Since both words have been in use as both adjectives and nouns, I would say they're interchangeable. In any case, I would rather call myself a Nepali.