How about the other job? Better possibilities for development there? Seems like it would be tough to (ever) get a promotion if you take the job at your current company. The question is, how long will it take before you grow tired if you take the new position at your current company?
While the promotion might give you a new challenge, it seems to me that what you really need is a new work environment. You have stagnated and the new job might just be the boost you need. The fact that it took your boss to hear about your other job offer to offer you a promotion doesn't sound promising either. Sounds like you should have been promoted a long time ago, but that he thought he could get away with not doing it. You shouldn't stay just because you feel safe there.
