Distressor: Compared to Fatso, has far more flexible compression (attack, release, ratio), "harder" somewhat edgier sound (in a good way too). Cool side chain for controlling low end frequencies (detection or audio HPF). You can turn off the harmonic distortion options if you want and get it relatively clean (never "hi-fi" though). More expensive for two units than a Fatso.
Fatso: Compression not extremely flexible, although it works about 40%-50% of the time. Silkier 'softer' more laid back sound as compared to Distressor. Can't turn off harmonic distortion generator function, as it always remains on, even if subtely. "Warmth" function can make a cool de-esser. Good deal for two channels.