As of 1.2, some features are mutually exclusive between worlds. In the case of ores at least, these seems to serve as functionally identical replacements. However, a Crimson biome gives you the chance for a Panic Necklace (and the upgraded Sweetheart Necklace) which does not seem to have an equivalent version in the Corruption.
What items, if any, are truly mutually exclusive in function between worlds?