I'm not sure I know what an "exec" course is but wonder why you would restrict yourself to an either or scenario. Surely there are so many other options some of which would be more preferable than others depending on the other selection criteria.
exec = Executive Course, or a course that's shorter than a standard golf course in one or more respects. It's usually comprised of more par-3 holes than are found on a typical golf course. in other words, an exec course is a par 3 course with a handful of par 4's and par 5's. In which those par 4's or par 5's would often be considered on the shorter length when compared to similar holes on a standard golf course.
Exec to 9 hole routing are not the only two option. I offered them as anchors on a spectrum for discussion and not a limitation of 2 choices.
And why does a 9 hole course have to be 3,400 yards ? Concentrate on the fun factor and forget the numbers.
Once again that was a reference for discussion based on what appears to fit within the property confines. 3,400 yards would represent an above average length 9 hole course and a value for consideration by the retail golfer.
Those that participate on this board have a much greater appreciation for atypical golf than the average retail golfer. If the course was positioned towards the average retail golfer, what would they care about on paper to persuade them in playing the course the first time?
Elements such as 9 holes vs 18; par 3 vs exec vs standard; yardage and par are all criteria they are familiar with. Even then they will weight those criteria differently when making a playing decision. That does not take an atypical execution off the table, i.e. 12 holes, but it can make it more challenging to market such a facility to the average retail golfer who is not familiar with the idea. For example, if 9 holes cost $30 and 18 holes cost $50, what should 12 holes cost in their eyes?