Nightfall said:
Favored Soul makes little sense to me IF you have a world where gods already grant spells OR you can use Mystic from DL in place of it.
In any event, if you want a powerful Sorcerer, Bloodline of Power, Blessed of Mesos will make you believe in the power of sorcery yet again.

But that's just me.
Favored Souls make perfect sense in a world where gods grant spells; one of the classes features surrounds a deitys favored weapon. Unlike clerics, though, favored souls are touched by a god essentially since birth or so, with a natural attunement to their faith and deity.
Also, any class can have a broken prestige class brought up to make it seem spiffier; that doesn't do anything for the base class, though, particularly if a character concept doesn't fit the prestige one.
Anyway, in regards to the general topic at hand...
When comparing the favored soul to the cleric, I'm actually inclined to say it's not too much better than the cleric. While turn undead in and of itself isn't a major thing, in light of the various divine feats that work off of it, the cleric always seemed the more attractive class to me. At least when coupled with the dual ability score necessity.
In comparing it to the sorcerer, though...then the favored soul starts to shine more. However, as others have said, I think the problem is more with the sorcerer.
A good example of this, in my opinion, is the battle sorcerer from Unearthed Arcana. I compare the normal sorcerer to that and see the d8 hitpoints, the martial weapon proficiency, the increased base attack bonus and the light armored spellcasting and think that it more then makes up for the diminished spellcasting ability. Perhaps this is for lack of seeing the battle sorcerer in play, but, still. In comparison to the battle sorcerer, the normal sorcerer seems lacking.
And even if the arcane spellcaster is an eggshell with a hammer, perhaps it's a sign that the hammer should be toned down a bit and the shell made a little more resilient.