basically the cathedral has all of the holier grail settings, except for the gate feature (and i can’t remember off the top of my head if it has flerb). the cathedral also gives you way more forms of reverb – the holier only has spring, room, hall and flerb.

another big difference is that with the holier grail, you get preset switches to select between short / long verb, whereas the cathedral gives you knobs to adjust every aspect of the sound – including predelay. and you cannot save presets with the holier grail.

and the cathedral is smaller, and roughly the same price.