There is no choice in that matter: Saint Petersburg IS the pretty, decadent city to go to SEE and enjoy old, classical stuff in Russia.
The Louvre is THE museum, the Prado is THE collection..: a collection's worth is NOT derived from some general, more or less general concept, like its quanity and/or variety, but from the coherence and degree of completeness in the development of a concept derived from, or present in the items of the collection itself... the Louvre's idea, at least about twenty years ago, was to "have it all"; the Prado's collection cristalized around the royal power in the era of the great European masters and rediscovery of ancient classical art, and if it possessed more Dutch classics, and maybe a couple more old Roman copies now in Rome and Naples, it would be, in terms of art and not just crafts, even above the Louvre, for all the difference in number of items.