I think both are good - I was trained in choirs so I believe any groups of singers should blend together so that they sound like "one voice". I do however also think that a song leader should stand out enough that the congregation knows who to follow. Otherwise they can end up following a back-up singer into their harmony. Not that there's anything wrong with that.
Part of a good mix is your sound guys knowing who is leading and how to mix the voices you have together.