Advantages of dual degree: structured training, more years of mentorship and experience, no debt (provided you go to a fully funded program), more experience in scientific writing and hopefully (if you pick the right program) grant writing, integrated scientific and medical training to you learn how to think critically like a scientist and methodically like a physician concurrently and in a complementary way.
Disadvantages: 4 more years of schooling (read: significantly less $$$), no guarantee of actually being able to do both science and medicine once you finish your infinite training, might have to do a fellowship or pursue additional science training after residency for a junior faculty position anyway (which you could have done as just an MD), and my fav one from Neuro's blog, PhDs wont think you are a real PhD and MDs won't think you want to practice medicine or know how to deal with patients. MD / PhD admissions are much more competitive than MD-only admissions. The "least competitive" program I plan on applying to (non-MSTP) has an average accepted MCAT score of 35 and GPA of 3.8 and matriculates 5 people. The majority of the programs I'm looking at have an average MCAT between 36 and 39 and GPAs of 3.8 and 3.9 and matriculate about 10-15 students per class. This is not to dissuade you because anyone with a 3.6 / 33 / 2yrs + of research is competitive and fewer total people apply but it is a very competitive process (lots of self-selection). The good thing is that typically if you still want to pursue the dual degree and enter an MD-only program there are other paths available to you or you can transfer to that school's MD/PhD program via internal application.
I would have to sit here for hours to explain every single pro and con. Long story short: It's complicated but it's a rough life if you want to do science no matter which path you take right now. I recommend you watch the following videos:
Supplemental reading: Go to the physician scientist forum and read the sticky threads on MD vs. MD /PhD vs Phd--->MD physician scientist pathways, read the MD/PhD admissions sticky, and then read the following threads:
http://forums.studentdoctor.net/thr...the-beginning-of-my-md-phd-experience.725387/
http://forums.studentdoctor.net/threads/academic-salaries-in-different-specialities.1170716/
http://forums.studentdoctor.net/threads/how-to-fix-mstp.900721/
check out neuronix's blog at
http://www.neuronix.org/
dated but still has informative reading on it.
