Mostly I have sold plots, its true that I do not tend to have or offer many pre-builds but most people prefer to either go for the cheaper option (builders) and/or build their own catered to their needs as they perceive them.
I would do just a few pre builts for the most part people prefer to design their own house, and with plots they can have a farm.
I prefer prebuilt houses cause i can't build a good mansion for the life of me. Also me being to lazy to build it comes in as a factor.
Prebuilts are only good for builder who have a hard time getting started. If you're going to make one I would suggest putting a small sugar cane farm on it saying they can start making money from that. Then message random builders you see in global chat and ask them if they want it. Make sure you make the prebuilts on the smaller plots so you don't lose money. Use every plot bigger than a 15*15 empty as those usually sell to residents.
I don't sell plots, only pre-built, but I would say that plots are probably more in demand. I prefer pre-builts as I am very particular about how my cities look, and prefer to build structures myself and then sell them. In my experience I tend to sell to more experienced players who are more interested in just having a nice house in a nice city for trophy reasons I suppose. It's much easier just to sell plots to builders, but if you're willing to put in a lot of time then pre-builts certainly have their benefits; mainly that you can make more money from them.