Carol singing and mass to go to in Park Slope?

Have you tried St. Lukes and St. Matthews Episcopal Church. It tends to be on the "high" side and the singing is wonderful. The choir really has it together and the organist is really good. The American Episcopal Church is in between the Lutherans and Catholics.
Location is right off Atlantic on Clinton. 520 Clinton:

http://www.stlukeandstmatthew.org/events/#Choral

I am just back from choir practice at St. Francis Xavier, on Carroll and 6th Ave. I can tell you we have all the good stuff planned for Christmas Eve. You just have to be willing to come at 11:15 p.m. for midnight Mass. But it is all there - the standard carols plus really cool performances in French and Latin. And (from what I could see as we left the space) loads of poinsettias. Pageantry, music, great florals. What else could you ask for?