I had always use the buttercream to make a little barrier to prevent the filling from ruining the fondant. but I notice sometimes that one day latter the cakes have a gulp or bubble where the filling is. I don't know if I should do what I heard on a tutorial today to use a rope of fondant instead of the buttercream? The person was saying that if we use the rope of fondant will would be better to prevent the bubbles you get when the weight of the cake could make it sag... Anyone knows which is better if buttercream or fondant??

I always use buttercream, but very stiff buttercream. I mean stiff enough that you could practically roll  it into a rope. It's always my least favorite part of a cake to do because it's so hard to squeeze,lol.

You could use ganache as well. Ganache makes a good dam . 

AYour cake is s settling read this thread about cake settling cakecentral.com/t/633571/my-newest-trick