For someone who has never seen SW (or for someone who has, for that matter), I can't imagine a 6-movie marathon. That would be exhauting to the point of torture. I remember back in about 1985 when a local theater did the 3 movie marathon (4,5,6), and that alone is 9 hours and by partway through the 3rd movie I was mentally done and so was just about everyone else in the theater. You're talking 18 straight hours which is borderline unhealthy.
If you mean a 2 or 3 night "marathon", I still think 456123 is the most logical sequence, because it is the order they came out in theaters. No reveals are spoiled, and the progression of movie-making techniques and styles would be less jarring.