Not all Tau. Basic Tau that fight for the greater good you're 100% right with, but let's not forget Commander Farsight and his bunch.
OT: Depends on the situation. Close range, ODST. Long range, Tau.
Edit: Forgot the why - ODST are trained for melee combat as well as many other things - they were supposed to be the closest thing to an un-genetically modified SPARTAN that the UNSC could make, while the Tau are experts at sniping the shit outta anything from as far as possible, they literally cannot fight close range.