Location for Save the Dates

I would probably use the ceremony city.

The ceremony city

I would use the ceremony city. People want to know where they are traveling to. If they are neighboring, it isn't that big a deal as I'm sure it won't significantly impact people coordinating travel.

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Both: Ceremony in_____
followed by a reception in/at venue, in ____.
I would find that most helpful for booking lodging, car rental. Of they are a mile apart it does not matter. But most likely yI either want to book between the locations, or if I will visit friends or family while there, I want to know the 3 locations so I could come early and stay between friends and ceremony, and 2nd night walking distance to the reception. Otherwise, I will be on the phone to you.

I would use the ceremony city

If there is enough room to list both cities ("Ceremony in [city], Reception in [city]"), then I recommend that! Otherwise, if you are only able to list one city, I would list the ceremony city.