So long as the Russian decision makers believe they will be hit with nukes if they use nukes themselves, it doesn't matter precisely which target — the point of MAD is a deterrent that prevents the nukes getting used in the first place.
If the decision makers think "we can use as many nukes as we want because we have a huge stockpile and can level anyone who even tries to respond in kind", we're already on borrowed time.
Equivalent retaliation would be targeting a city of some of the Russian allies, but it's not possible since Russia doesn't have allies ;)