In Pokemon Go, which are more rare: Stardust or Candies?

Stardust is shared amongst all of your Pokemon, and upgrading a weak Pokemon can prevent you from upgrading a stronger one. However Candies are per Pokemon evolution chain, which means the rarity completely depend on the rarity of the Pokemon.

It's generally best to save your Stardust for your stronger Pokemons (or ones you really want to play with). Especially as you'll find higher CP Pokemon as you level up.

It entirely depends on the rarity of the Pokemon you are trying to max out, but generally id say stardust will be your limiting factor. If you are trying to max a more rare Pokemon you might have trouble finding others and therefore be limited by candies. If your Pokemon is rather common you shouldn't have an issue getting as many candies as you need.