The difference is that one is natural English and the other is not!
The first is a natural comment. In spoken English, it is normal to use the present continuous to refer to a planned or schedule event in the near future.
The second is completely unnatural, because 'is to' + [infinitive]' is a very formal construction. You might come across it in written texts, such as a news reports, business letters, contracts and so on. On rare occasions, you might use this in formal spoken language. A native speaker would never use this construction in conversational English.