There are two answers here: 1, If you are studying for an exam AND 2. What applies in the real world In relation to number 1, we usually teach students that Present Continuous is used then there is something that has happened that means the future event is pre-planned and organised (I have bought an airline ticket and therefore I'm flying to Paris tomorrow) AND 'Will' for things we have decided but haven't done anything about yet (I don't know what to do this weekend. Hmm... I know...I will fly to Paris tomorrow if I can buy a ticket.). In a way we teach students this because it is easier for them to understand, and maybe also easier for us as teachers. It's nice to have some rules. BUT in relation to number 2 (the real world) native speakers mix these two forms all the time. The problem is that grammar and reality are not the same thing. The way we teach Present Continuous is as though the future is definite...and the future is never definite. (I think that I'm flying to Paris tomorrow because I have bought a ticket, but maybe the flight will be cancelled or I will be ill?) AND then also lots of people think that because they have made a decision then the future thing is almost certain. So you hear people saying something like "We're getting married." but if you ask them when/where/etc they have not made any plans. It's simply a joint decision that they have made (maybe only a few hours before)...but in their heads it is sort of definite, and they don't care about the grammar rules. SO my advice is that students of English should learn the rules (as per number 1)for exams etc, and then not worry so much about those rules (and accept number 2) when having real conversations