Cual es tu nombre is Formal, is actually more formal Cual es su nombre if you add the "tu" is a bid more informal but if you are meeting someone or something like that I would say, Como te llamas? if I'm a personal banker and setting up an account with a client but I want to be friendly but polite I would say Cual es tu nombre?and if I want to be super formal I would say Cual es "su" nombre?