As a native U.S. speaker, I would call these "mostly" interchangeable in common use. I see this literally as two equal options of future tense.
Here's the only reason I say "mostly" (vs. 100%):
Say you were at a wedding, currently witnessing it, and someone important is calling you on your mobile phone and asking what you are doing right now. (You would take this in the lobby/vestibule, of course! :D )
At that moment, if you say "Emma and Nick are getting married!" you are speaking in the present tense, informing the caller it is happening literally right now. In that case, "going to get married" would not work and they would not be interchangeable.
But since I had to contrive that hopefully rare experience, it is merely technical. Common usage would mean "future" and both work perfectly well with no differences.