An Archivist theoretically has access to every divine spell, but in practical play it's impossible to have that. Clerics, however, aren't limited by having to scribe spells. Having to buy scrolls to learn spells can eat up a lot of money, and keeping the prayerbook safe might also become an issue.
Clerics also have Turn/Rebuke Undead, which an Archivist doesn't have except through Sacred Exorcist and potentially other PrCs. Turn Undead has the potential to fuel a lot of things, including Divine Metamagic, which means Archivists miss out.
Archivists are also 1/2 BAB and d6, the same as Cloistered Clerics. I'd suggest getting a hold of the book if you're going to comment on it.
Both classes are quite powerful, enough so to break the game in many ways. When you can literally break and mold reality, power level tends to become a non-issue. It all boils down to playstyle preference at that time.