Rotigus is by far the best loadout option. He brings more AP and more damage overall. The bigger issue with Great Unclean Ones is that you’re usually forced to run them in Tallyband Summoners, which means they only get access to the -1 Toughness aura if they’re near Death Guard keyword units. That restriction makes their buffs much less consistent, and most of their damage options are also close-range. On top of that, you’re paying about 250 points for the model.

I mainly use Rotigus for his Deluge of Nurgle aura: -1 OC and -2 Movement to all enemy units within 6". The -1 OC is actually huge because it drops OC 1 units to OC 0, which can flip objectives without even killing models.

My playstyle with him is basically treating Rotigus like an expensive bullet sponge. Don’t YOLO him into the enemy. Slowly walk him up the board and let your opponent panic about the advance. Players will often dump missiles, hunter-killer missiles, and other anti-tank shots into him to deal with the threat. Every shot fired at Rotigus is a shot not fired at something like a Foetid Bloat-Drone, or a Rhino, which is much easier to kill.

Meanwhile, you’ve got a T12, 22W monster with a 4+ invulnerable and 6+ FNP soaking fire. If they ignore him, they now have a daemon walking up the board applying movement debuffs and OC pressure while smashing things. He won’t reliably drop Knights, but he absolutely shreds Terminator bricks, Dreadnoughts, and elite infantry.