Aye, clerics are definately the stronger of the two, overall, but not by much. Though somewhat better in combat and less restricted in spellcasting (mechanically speaking), clerics lack the familiar, bonus feats, and larger spell list of wizards, while they likewise lack the familiar, superior spell list, and superior spellcasting of sorcerers. The ability to turn or rebuke undead is also much more specialized and restricted than the benefits of sorcerers/wizards. Still, in this way, clerics are just a little bit more powerful.
Arcane magic and divine magic, I believe, are quite even - divine magic is restricted mostly in roleplaying ways, not by armor or lots of expensive spell research, but has slightly more limited uses, and the few divine combat spells are usually inferior, while divine spellcasters can never have such options of powerful specialization as do arcane casters. Druids, for instance, are intensely focused on nature spells, but most of their spells that even vaguely resemble those of mages are obviously inferior, either in assigned spell level or in simply raw power or utility (i.e. hold animal is much more restricted than hold person, produce flame is less potent than flaming sphere, wall of thorns is inferior to wall of iron, etc.), and they lack the spontaneous healing or inflicting of clerics. Divine casters only benefit from the less restricted and more defensive nature of their spells, but even then, they have roleplaying limitations to balance things out.
When it comes to spellcasting, the cleric and wizard/sorcerer are quite different in focus and capability. The cleric spell list is focused almost exclusively on healing, divination, "buffing", and support spells. They also have just a rare few generic combat spells, plus a few alignment-specific spells, a few spells for fighting or consorting with outsiders and undead, and a few generic utility spells (Make Whole, Helping Hand, Air Walk, Word Of Recall, etc.). Clerics are overall quite superior in defense, and in divination, support, and utility. Clerics are always useful on any given day, not just when adventuring, battling, or whatever. However, clerics do have some shortcomings as well. They have little stealth capacity whatsoever, few mind-affecting spells to manipulate others with, few attack spells that aren't melee-only, few and limited travel spells, hardly any deceptive spells, hardly any form-altering spells, inferior magic dispelling and counterspelling capacity/usefulness, slightly more-restricted creature summoning/calling capacity, and attack spells that are very limited in variety, usefulness, or spell-level-dispersal (i.e. mostly just fire-based, bludgeoning, piercing, slashing, or negative/positive energy)
Anyone can benefit from a cleric's spells, from the farmer to the blacksmith to the nobleman to the king to the guardsman to the adventurer to the beggar. A cleric can call upon his or her deity's guidance, heal friends, repair almost anything, locate and get rid of pesky poltergeists, exercise a demon, cure any ill whether physical or mental or spiritual, thwart invisible thieves, clear away a trap-filled problem spot, solve a crime investigation through speaking with the dead or a deity or interrogating with truth magic, curse infidels and criminals, convert rivals to allies through diplomacy and religion and healing and magic, raise the dead, prevent or mitigate natural disasters or plagues or famines, create a tasty banquet, support starving folk, purify and cleanse a site, put the undead to rest or command them as thralls, thwart a maniacal wizard, calm a barbarian warband, bring down rains for the crops, destroy enemies, and even perform remarkable miracles.
Wizards and sorcerers on the other hand, have obviously superior versatility and raw power, but less utilitarian or support spells. Their spell list is larger and more varied, with many spells of each school (though Necromancy, Enchantment, and Abjuration kinda get the shaft with mages). An arcane spellcaster has a great many capabilities no divine caster could duplicate short of high-level Miracles, and arcane spells often have greater raw power and combat usefullness. However, mages have fewer and generally less useful defensive spells, with a few exceptions, and also universally possess inferior physical capabilty, with lower hit points, armor use, and melee capacity, further reducing their defense. Mages also have no access to the Death Ward spell or its ilk, and death spells are often Fortitude-based, as are most of the powerful offensive magics and hazards (i.e. poison and disease), along with Reflex, both of which are a mage's weakness. Arcane casters have practically no healing/restoring capacity, aside from the few healing spells Bards slowly gain access to, and a few construct- or familiar- restoring spells for arcane casters in optional supplement rulebooks. Mages have to rely more heavily on others in order to survive, and often it's the divine caster they most rely upon.
Overall, I say divine and arcane spellcasters are fairly equal, though divine casters have a slight general superiority over most arcane casters, and which is simply more readily apparent than the more subtle strength of a mage.
