The differences are that Rare Candy allows you to evolve a Pokemon even if it is that Pokemon's first turn in play, and you don't have to skip the Stage 1, although you can. Rare Candy does seem to make Breeder look like a joke, since Breeder can't do any of the above, and can't do anything that Rare Candy can't also do. Of course, you could always have 4 Rare Candy and some Breeders in your deck (if it's Unlimited), but why would you want to?