He got offered a promotion where leaving, because he decided to leave. That promotion was always available but only offered when was necessary. They likely were having that labor for free without the promotion and just riding on his goodwill. That said now the decision is simpler, why would you like to stay where you were not valued for your quality of work? Yeah the new job can be all kinds of different but is additional experience and OP indicates he loves the move. To me, this is an easy one.