Attending a church service in Paris; which one?

I think you mean St Sulpice. They have the big name organist. My 2012 RS book says 10:30 mass is followed by 25 minute organ recital.

St. Eustache, which is near Les Halles and rue Montorgueil, has one of the largest organs in Europe and there is a 30 minute concert at 5:30 Sunday evening, followed by mass.

We attended mass at the church that was just down the block from our RS Hotel http://www.paroisse-ndc.com/.
At this church there was no choir but the organist was over the top. I am not sure that you can go wrong in Paris. The quality of the pipe organs and the expertise of the organists seem to be extroidinary.

In 2012 my wife's choir performed at the London Olympics, but first did a few days in Paris and performed at a couple of venues, including Eglise Madeleine. The acoustics there were astounding! As was the church itself. But I nearly cried when I heard the beauty of the music they were making. They have both a grand organ and a choir. The choir was on its summer break, which opened up the spot for my wife's chorus. I plan to go back to Eglise Madeleine just to inquire about choral performances. They have a web site here: http://www.eglise-lamadeleine.com/la-musique I highly recommend that you see whether they have a performance while you're there.