The 8 week/16 week plans really are minimal type plans for climbers who are close to their goal date so not many weeks to train. As such they do not allow the time for a big base of fitness to be built up.
The most complete and properly structured program is the 24 week.That will take you through a periodization process as described in the book and for sure give the best results.
If you don’t have a trip planned allready and just looking to improve fitness, with the 14ers in mind, going through the 24 would be better. And if a trip comes up you can incorporate that into the program with some adjustments ofcourse.