For me it's phaser because the music my band plays is softer.
My old school sensibilities yell "PHASER!", but I think a flanger offers more color and tonal variety if that's what you're shooting for. If you want to showcase the bass, I think phaser is a little too subtle unless you wind it out, but then there's the danger of the effect overshadowing what you're doing musically. Subtle flanging can really complement a nice solo or add a warm layer of smooth to the groove like a phaser can't.

