I think it is the exact opposite what you said. The most difficult part of creating a fictional track is not about creating realistic and perfect road cambers, correctly engineered ground fillings and matching some ideal only a real circuit could be. The most difficult part is figuring out, understanding and creating the imperfections what create real race tracks. Very few tracks have perfectly round corners, perfectly straight straights, perfectly smooth camber transitions, straight white lines, bumps or any other kind of perfect anything. Very little made before computers is done to an accuracy of even one meter. Not even close vertically. Every real track is full of these flaws and that is what makes the best real and fictional tracks really good. It is all these imperfections.I voted real but I have enjoyed a couple of fantasy tracks here and there. Dave Kaemmer captures my thoughts pretty well in the nurburgring promo video iracing put out: real tracks are made by expert teams of surveyors and engineers. It's hard for a lone or couple of fantasy track modders to beat that for creating the right road cambers and gradings. Obviously not saying that all real tracks are winners but nothing in fantasy category compares to the best of real for my 2c.
I choose both. Especially if the fantasy circuits are made like they were real. Toban, Essington, Mills, Aosta come to mind. And it's a lot of fun to design the fantasy circuits as if they were real, while still keeping a bit of unique idea.