I've played both, but my experience isn't balanced - retired my druid (and the Prairie Chicken that was her constant companion) at the tender age of 23, while my Paladin main is raid-ready on two specs and heroic-capable on the third...
Druids get their specs differentiated much sooner and much more clearly (what with the forms and all) - while Paladins don't get any real spec-defining talents until 30 and the specs don't really get all their flavor until the 50s. And they keep the differentiation into the endgame, since the forms are defined by the mechanic they use (rage vs. energy vs. mana) - all Paladin specs use the same mechanic, and the difference between the specs is deep but subtle. Paladin specs are defined more by their gear (another subtlety) and while many shared skills are baseline and not talented, successful play of a Paladin spec requires just as much distinction in viewpoint as a Druid.
I carry around my Ret gear as holy (and vice versa), even before 3.1. But if I throw it on and try to smash faces it doesn't work, since Holy doesn't have the spec-defining talents to implement the combat philosophy that leverages Ret gear. And likewise, if I'm DPSing away and the healer DCs, I don't necessarily want to be a faux-Holy, I want to pick up the slack like a Ret would.
That said, if your main is a rogue and you're looking for an alt that will give you a different experience, I'd say Paladin might be better. This is since feral DPS is much closer to a Rogue than either is to a Ret Paladin. Don't worry about relative power levels and not being invited to endgame groups - by the time you get an alt to 80, Blizzard will have rebalanced things at least twice.