I like using the phaser for subtle, creamy modulation. The phaser can also take a mono input and output stereo - there's a secondary output on it that inverts the phase of the primary input, so you get wonderful swimmy stereo effects, which you might pick up on in
this track.
The Cluster Flux, on the other hand, makes wonderfully abrasive sounds. I'm on a self-imposed spending freeze for the next few months, or I'd have bought the one that showed up on the Philly Craigslist a few weeks back. I would use the ClusterFlux on, for want of a better term, solos. When I need a melodic line to tear and burn. It's a lot of sound for an effect, which IMHO should be used sparingly. Given the choice of the two, I think I'd take the phaser first.
It can do weird things too.