The Open Heart has been around since 1976; while the key collection came out in 2008. Imho, the key collection is not as classic as the Open Heart. Both can be worn daily.
I would have picked the heart last as it looks young to me - maybe you could share what you're looking for - is it the most classic, the most classic Tiffany, the most versatile, the one you'll be able to wear for the longest?
These are all great necklaces but either the key or the letter "a" pendant (unusual and gorgeous) gets my vote. I think you should ultimately pick the one your instinct says is the one for you! (And then let us know which one that is!)