First, know that in the future you will regret (on some level) this decision---no matter which one you make. There will always be bumps down whichever road you choose and you will idealize the path not taken as being better at that moment. In this specific instance I think either path is a good one, so I don't think you should think of this decision as a monumental one in your life--take some pressure off of yourself.
With that in mind, here's what I would do. Pick a percentage raise you would be happy to stay in this job for. Maybe its 10%, maybe 15%, that number is up to you. Next, add 5% to it. Let's call that Stay%. Now, take the other job. I literaly mean accept their offer and tell your employer you are leaving. If your current employer position offers you Stay% or more, then stay. If they say nothing or offer less, leave. I wouldn't negotiate, I'd just have a solid plan for how to react if they do offer more money to stay. Treat life like a chess game and think 1 step of the other guy--plan your counter action to their possible reactions before you make a move.