First, there is never such a thing as "guaranteed" publication. There are always things that can potentially get in the way, many of which are not in your control. To present two options as, "1st author publication" vs. "time to study" is very dangerous.
As someone who lived int he lab for most of his undergrad, there is no diminishing of publication. The act of going through the process alone is a worthwhile exercise, even if the content of the paper is complete garbage. First authorship takes that a step further and states that you took the lead on a project and took an idea and made it into a product worth reading about. It is generally a mark of a reasonably high level of accomplishment. Keep in mind that you should have a LOR talk about the project, you should at least mention in your PS and be ready to defend it in interviews to maximize it's impact on your application.
All of that having been said, this question is more about YOU than it is about the research. Have you already burned out on the research front? Do you enjoy it? Is it something that at the end of the day you would do for yourself or is this just for the publication recognition? Let me say up front, there is nothing wrong with getting publications for the sake of getting it. Just realize that it comes at a cost and it isn't always what it is cracked up to be. Publishing is great and will make you attractive to places, but if you are working just to get the paper out and not to learn something or because you are interested, it will show in LOR, PS and interviews. That experience while on paper will certainly help you, will have diminished returns, especially if you are aiming to have this be a focal point of your application.
Do not take a gap year to study for the MCAT. That is just wasting time. Also, this whole, "being mentally prepared for medical school" is hogwash. Enjoying undergrad doesn't help you do better in medical school. Enjoy undergrad because you should enjoy life and the journey that is pre-med/MS/physician. You do not have to do research to get into medical school. But, if you enjoy it, you should do it, and do it well. You will learn something useful and you because of that you will be a more attractive applicant.