My PD experience is this:
4 year is slightly more competitive (due to the number of applicants) and therefore I see more applicants to my 4 year program with internships. It's a matter of numbers. For me, if it comes to ranking an applicant that has an internship versus a FOS applicant the internship applicant wins every-time. All things being equal, I am assuming the program will need to put in less effort to teach basic skills to a non-intern resident. Although I have been VERY wrong in the past with this assumption.
I would say this to anyone who does not want to put in the "extra year/time" for an externship. I would definitely not make that public knowledge. As a PD I want to know an applicant will do anything to obtain the position and if the prospect of an additional year is an issue then why apply at all and just go out and make money as a GP DDS immediately. Now, I certainly understand the desire to obtain a position and complete residency as quickly as possible, again, it dollars right?
I have heard that residents who have completes an intern year are better prepared to handle residency. I call BS on that one. Over the last 8 years of my academic career it is nearly impossible to tell who will be prepared and those who will struggle, regardless of the intern or 2 intern years. There are some really crappy internships out there that do not prepare you for time in a hospital. And I also think there are just crappy applicants as well. There I said it, some people who desire residency, for whatever reason, just are not a good fit. Unfortunately, there is not resource or data that looks at success rate of matching for each internship. Unless someone knows of a resource that I am not aware of.
My personal experience is this: I did an OMS internship at a VA Hospital that had zero trauma. But, I was prepared and ready to be in a hospital day 1 of my residency. I understood the nuance and inside workings of a hospital. How to page, how to be consulted, how to consult other services, how to write different type of hospital notes, how to round on in-patients, hierarchy of the OR, etc. The list goes on. Can these skills be taught early in residency, sure. Dr. Krishnan at Cincinnati has a "boot camp" for 1st years and interns prior to July 1st to get new residents up to speed. A wonderful idea that I have been trying to emulate but have had mixed success.
Bottom line, Dental school provides no preparation for residency whatsoever. Not even a little. So information/skills have to be learned individually and that is usually not something dental students are good at. They have been spoon fed information in PPT format for 4 years. So independent learning is not a skill that most applicants possess. Applicants who are self starters, self motivated and self learners will do well. Those who think residency is like dental school typically do not do well.
Hope this helps.