Divine prestige classes are, for the most part, less stellar than arcane ones. Incantatrix and initiate of the sevenfold veil have no competition among the prestige classes available to clerics, and druids rarely want to multiclass at all since almost nothing advances everything they have going on.1
For the most part, a good cleric prestige class is one that fully advances cleric spellcasting, and gives an extra domain. That’s what most of them are good for—church inquisitor, contemplative, and divine oracle fall in this category, for examples. Aside from that, there are a few decent “warrior” prestige classes—bone knight, ruby knight vindicator, sacred fist, maybe prestige paladin—that give useful things for not too terrible a cost. Beyond that, there’s a smattering of other full-progression prestige classes, which by dint of being full-progression, aren’t bad, like sacred exorcist or thaumaturgist. I’ll throw in malconvoker, sovereign speaker, and walker in the waste as examples of “specialty” prestige classes that could be useful for particular goals.
So I just named thirteen prestige classes. Only one—sovereign speaker—requires that you already have a domain, and it can be any domain at all, so gotten easily enough from another prestige class. Bone knight and ruby knight vindicator require turn or rebuke undead, which you could get from sacred exorcist. Church inquisitor, sacred exorcist, and walker in the waste require the ability to cast particular spells that you might not otherwise want, which might hurt as a spontaneous spellcaster with limited spells known, but you certainly can. (Divine oracle’s requirement of two divinations probably shouldn’t be onerous, and it’s not as though a malconvoker would ever want to be without summon monster III or a thaumaturgist would skip lesser planar ally.)
My point is, there aren’t all that many great options, and none of them stand out as just being impossible for a favored soul. But you have to jump through hoops to get them, while a cleric gets access far more easily.
The flip side is... there are prestige classes the favored soul can take that the cleric cannot. Not, ironically, due to the spontaneous casting—even the regular cleric gets that (it actually uses the word “spontaneous” while favored soul does not), and no prestige classes use the “without preparing” wording (which is what favored soul does say, and which a cleric couldn’t claim since their spontaneous spells have to be cast from slots already prepared as something else). No, the favored soul feature that opens up some unique options is the Weapon Specialization feat at 12th level, while a cleric could never get it without multiclassing to fighter (or something that counts as such). The feat itself is awful, and very, very few things require it, but those things are technically available to the favored soul and not the cleric. The only such prestige classes are the master of chains (Sword and Fist), the planar champion (Manual of the Planes), and the warmaster (Sword and Fist). They aren’t terrible classes in general, but mone of them advance spellcasting, so they certainly are terrible for a cleric or favored soul (and really they’re not great). So while this is something “unique” for the favored soul, it doesn’t matter because it’s useless.
Quite simply, the Unearthed Arcana spontaneous cleric is vastly superior to the favored soul. The favored soul’s particular class features are mostly garbage—poor bonus feats, tiny amounts of energy resistance, and the one great thing—flight—happens rather late. Two or three domains and turn/rebuke undead is just better than anything the favored soul gets. The favored soul gets more spells known and spells per day, but their 2nd-and-higher spells are delayed a level, while the spontaneous cleric’s are not. And the favored soul requires Wisdom and Charisma for spells, while the cleric just needs Wisdom. So the spontaneous cleric’s spellcasting is much superior to the favored soul’s. And, as a cherry on top, of the better prestige classes available to both classes, many will be more easy for the spontaneous cleric to enter than for the favored soul.
And, while we’re comparing these, let’s not forget that the regular cleric is greater still. Spell preparation is a huge advantage, and spontaneous spellcasters are greatly limited. There are several prestige classes that are much more painful for the spontaneous cleric or favored soul to enter than they are for the regular cleric.
- The one prestige class that does advance everything a druid does—Faiths of Eberron’s planar shepherd—definitely is competitive with incantatrix and initiate of the sevenfold veil. But this question isn’t really about druids.