Hmm. difficult question. Most of the man man sites I don't like, the material is hard on tires, knees and generally they are made to "break" vehicle with stupid climbs and cut backs and drop off's.. The one in Columbus is somewhat more realistic but also has some of those traits. And scattering around some real rocks with dirt and such was interesting - I like it better then others.
The stadium concept has merit the problem is that the spectators are farther away from the courses then at a natural event.. while some of the promotors use more sense when roping off a course the idea of being able to watch the event from right next to the course instead of from behind a fence is more attrative to many of the current spectators.. but depening on some of the future thoughts there may be a way to build man made courses and still allow the same type of access (safety is a mjor concern).
Natural or natural like man made would be preferred by most I believe but much of it depends on the promotor, the thought behind the course and the local alternatives.