It's tough to say. Depending on where you are within the regions you may hear different greetings, or perhaps even multiple at the same place. At least in western Germany we rarely, if ever at all, use Grüß dich, although the elderly may still use it. And to answer the main question: Grüß dich, Grüß Gott are standard greetings and thus can be interchangeably used. (Personally I'd not use them too feverently, as they aren't all too commonly used.)