It's more important to have good numbers and apply early. As such, neither will probably be printed by the time your fate is sealed. Don't do research as a means to an end (i.e. medical school). Do it because you enjoy it. Otherwise you'll be exposed upon questioning.
In the context of an application to medicine, it's best to mold whatever experience you have into an interest in the medical field. From an academic research standpoint, a first-author publication as a junior in college would definitely show ambition and is becoming more commonplace these days. A faculty member writing a book chapter with an undergrad (or even a grad student) would be unheard of at any of the institutions I've worked for unless it was merit badge literature sponsored by the Boy Scouts of America (that I have seen).
Either way, if you have that much time to dedicate to research, your interests would seem to lean toward a Ph.D. program or M.D./Ph.D. There's nothing wrong with that, but I'd like to see what other EC's you made time for.