A quick thought:
It would highly depend on how vital the information is that this publication carries to its readers. If the readers are professionals and this publication is a must-read in the profession, a strong case can probably be made for subscription only.
If we're talking about fluff reading by Joe and Jane consumer, well, that's not likely to be so successful as subscription.
As for Adsense, the more highly niched and specific the articles, the better Adsense might do. The deeper the content also, the better.
Do you have any metrics at all that help you make a business case for this? For example, if you can say that the free story is read by 50 times as many people as subscription stories, well, that might mean there's a huge appetite for the free stuff that is untapped. If it's only a three-fold difference, seems to me that it's probably a no-brainer to keep it subscription-only.
The more information you can muster, the better.