The difference lays in surivability. A lord w/o a daemon weapon will never, ever be able to compare to a DP. But the daemon weapon balances lower S attacks and survivability by having more attacks. A khorne lord w/ bloodfeeder (dangerous but fun, I run him in semi-competitive) will take 4 + 2d6 power weapon attacks w/o the charge. A tzeentch lord can shoot them up and then charge. A nurgle lord will kill MC's faster than a prince, and a slaanesh lord will wipe multi-wound squads like oblits, HQ's, spawn, ogryn with super ease. Prince has better survivability, and psychic if non-khorne. For an example, just last game my khorne terminator lord with a bloodfeeder and in a squad with 4 surviving zerkers (after a battlecannon shot) was charged by a prince, a full zerker squad, and oblits. My zerks were wiped but my lord took 14 attacks, killing 9 zerkers and winning combat for me 9 - 4. I forced 5 saves onto his last zerker, oblits, and prince. So in 1 round of combat my lord killed 10 berzerkers, wounded a prince, and wounded an obliterator. Already payed for himself and more.