Well the music dictates that to a large extent I would think. Music first, always. When I played with 100% passion, it wasn't as good for the music than if I remained semi detached. The recorder proved that to me. As far as power goes, I use only about 20% of what I have on gigs, which is fine here. I feel no need to blow everyone out. My volume is why I get hired, I can hold it back. It's a necessity in my little quiet world.
I'd pick passion if I had to choose. Passion can be conveyed at any volume. If I played with power all the time, I wouldn't be employable in most places I play.