Not entirely sure Machine. If I understand the way it works correctly, every client who logs in receives information from the database, meaning, the database will have to send the information about the "non-cooked" (not present on the "vanilla" version of the map) buildings every time he relogs to the server because that information is not downloaded as a permanent file but as a caché, meaning that when he logs out, there's no record of those buildings existing anymore.
This is the opposite to how the mission or script method works. When you put the buildings through the mission file or you add them as a script (like I do), the client downloads the mission.pbo when entering the server only once (per time you change it that is). All the custom buildings or additions are there, so what's left for the client is to run the scripts on his own computer so he sees the "non-cooked" material.
When it comes to performance, I believe that editing the mission is not only an easier and more customizable method than database additions, but also a much softer performance hit.
Thanks, even if there is no difference that just sounds better