From personal experience, I have found that breathing out of my nose and mouth at the same time creates problems when rolling to get my next breath because water tends to enter my mouth even though I am exhaling with my lips slightly pursed (complete open mouth exhaling is impossible for me). It merely is a mechanics problem with me, as much as some folks can do it. Because exhaling with my nose and mouth is not possible, I now strictly breath out of my nose. I am only relaying my personal experience, but in the end it is what you feel comfortable doing without taking in water while exhaling efficiently.