Morde's passive has 375 range, and does 8 − 25 (+ 30% AP) + 1% − 5% of target's maximum health magic damage per second.
Swain's ult has 650 range, and does 35 / 50 / 65 (+ 14.167% AP) magic damage per second, healing him for 20 / 35 / 50 (+ 16% AP) health per second.
So Swain's ult has nearly twice as much range, and heals him for anywhere from 60%-80% of the damage dealt. Mordekaiser with a Gunblade doesn't even come close to that amount of sustain.
Morde's passive gets out more damage per target, but Swain's ult hits more targets.
Morde's passive also doesn't explode at the end.
Swain's ult is definitely better.