A publication can only help you. But are you applying to an MSTP? If not then don't put too much stress into this research area. If you are then the publication would be more beneficial.
Publications are great, but they will have a better effect assuming you get to interviews where you can show them the actualy document if asked. Problem is getting an interview. Entries on a CV is acts as a good summary, but can be taken as a grain of salt without hard evidence.
If you just want to do a normal MD program you can do whatever you want that makes you happy. The #1 thing about research is to do something that you enjoy. Not neccessarily something that'll make you famous. Although relative to the thousands of applicants out there, having a publication does happen, and having a first-author publication is rare...and having a first author publication in a good peer-reviewed journal is even rarer. Problem is, as you get over each hurdle during the application, people with publications becomes more "concentrated". By interviews you will be going up against some amazing people, so publication or not, you will need something more unique about yourself to get you in into a normal MD program. Each person has their strengths so I can't say much beyond that.
From my perspective, aslong as you have some proof of your research, whether it be your publication, or a letter of recommendation, or research units, then you are FINE. Thats hard evidence. Its better to have all 3, but 1 or 2 is acceptable. Reason being, during primary applications, they will see your transcript, so will see research units. During secondaries they will see your letters of rec where the PI says you worked x-amount of time and did this and that, and finally during interviews you can show them your manuscript when asked. But as you can see, the whole publication bit doesn't even play a big role until interviews.
Ultimately the choice is up to you, I'd do something that i'm happy with rather than fame and fortune. I have found publications to be nearly a dime a dozen. I have always asked my undergrads to help out with publications, and over the years, 7 of them have already had first, second or third author positions in book chapters, manuscripts, and national presentations. So in my eye, they are not as rare as people make it out to be.