I think there is a 5th, and very viable, method which has been overlooked: I-II-IV-V-III-VI. Essentially, you get enough of the background through Eps I & II to see where events are leading. In this scenario Eps I & II act as "prequels" to a main story line: IV-V-VI.
But what about Ep VI, you ask? Well, it tucks very neatly between Eps V & VI. If you think about it, you have a "historical prequel" leading into a main story line, that has a flashback interjected right before the grand finale. So, just when we learn that Darth Vader is Luke's father, we flashback to see the whole transformation as portrayed in Ep III.
I have watched the entire trilogy as a marathon twice in this method, and each time it just seems to fit together neater and neater. The viewer isn't really left with loose or hanging ends, and all the storylines seem to tie together nicely.