In my experience, on overdriven sounds, a phaser sounds clearer but more extreme in the loop, and muddier but subtler in front of the amp. Do you want to phase a distorted tone or distort a phased tone?
This. When I use phasing it's for a pretty extreme "jet plane woosh" type sound, which is not achievable with the phaser pre-distortion, I can only get it by running it (MXR phase 100) in the loop.