We just loved our stay right beside the sacre cœur even the apartement points out to the other direction. It is amazing, very comfortable for two with a kitchen that has everything you need to have breakfast and to cook little things. Having two rooms makes it easy to have space for sleeping and getting up earlier than the other. The restaurants right next to the door serve lovely breakfast (be aware to wait a bit for your table) pizza, crêpes and wine. And in the morning you can visit the holy heart as if no other tourists would be there.
James holds a lot of books about Paris and the whole style reminds you always that you are in Paris.
I would always book this apartement again.