The "PX" dry is made from the same grape but is made into a wine called Montilla. At Alvear, they claim the flavor of the Montilla wine/vinegar is different (better!) in flavor than Amontillado Sherry because of the type of airborne yeast that resides in the moist soil. What we have then is a dry sherry-like vinegar made of a product that is a step above the base used for other sherry vinegars.
Sherry vinegar is our favorite kind of wine vinegar - tart and mellow, not too acidic but perfectly balanced for most any use. And having both sweet and dry to choose from...well, that's icing on the cake.