Usually, you would say 'I'm going to visit...' when you are discussing plans that are definite and were decided beforehand. You would usually use 'I will visit...' when the plans are not fully confirmed, or just decided at that moment i.e "I think that I'll visit' or 'In that case, I'll visit' - however these phrases are totally interchangeable in everyday conversation

Materially, none. I see them both as interchangeable.

When you use " I am going to" it is something that you have planned, it is something that you have in your schedule, and it is almost inevitable. but "will" is something that probably will happen such as a desire, or a promise but you have no a schedule for that yet.​

As a native speaker I can assure you that these two are interchangeable. Any 'difference' would be unnecessary hair-splitting.

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