From what I have discovered about Druid's, the best thing they have is the versatility they have, with Wild Shaping, Animal Companions, eventual 9th level spells, and the Planar Shepherd prestige class (from what I understand, this is the only one that progresses spells, wild shape, and animal companion all together). Clerics get domains, turn undead, and also 9th level spells.
In terms of the raw potential, I was comparing the two and I have come to the conclusion that Clerics can be strictly better than Druids (unless you do some Planar Shepherd thing where you can get infinite or multiple Wish spells?) and here's why:
Assume a Cleric takes the Animal Domain, he will have access to the 9th level spell Shapechange at level 17, and if a DMM: Persist is applied and used on self, the Cleric can now take on the form of any creature from Tiny to Colossal size of an HD up to his caster level for 24 hours, gain all their Extraordinary and Supernatural abilities, be able to use a free action every round (6 seconds) to change forms and heal his own hit points entirely, all while still being able to access all of his spells.
If a Cleric just uses DMM: Persist on a Summon Monster spell, then bam, he gets an Animal Companion (or multiple) that he doesn't have to worry about dying.
So now the only thing left that a Druid can do that a Cleric can't, is access the Planar Shepherd's Planar Bubble ability right?
Unless of course, you take the Divine Minion template (now you have access to the Wild Shape feature), and take the Spontaneous Summoner spell so you can spontaneous cast Summon Nature's Ally spells after accessing 4th level spells since the Animal Domain allows you to cast Summon Nature's Ally IV (or you can take that ACF that allows Cleric's to spontaneously cast domain spells), which enables you to qualify for the Greensinger Initiate feat. And now you qualify for Planar Shepherd as early as level 13th I'd think.
So upon reaching higher levels, specifically level 17, a Cleric can do anything a Druid can (apart from having an alignment restriction and having a different list of spells to access, but both get pretty kickass spells) and sometimes even better.