To get back to the orginal question....
what is better for a MS do do between first and seccond year:
Speaking as an MS with a masters in epidemiology and a phd in basic science,
... I would go with "clinical" research.
Why? Your liklihood of getting a publication are much greater. That is if there is a data accumulaiton protocol in set. i.e. if there is a protocol to get folks in for what ever... take a drug and their blood pressure checked, check urine out put, whatever....
if that is inplace, and no IRB approval is needed, then you may beable to get a small- failry reputable paper out, in three months.
The secret in short term, small, clinical work is getting the approval. IF that is set, and they are just waiting to get a med student to do the work. My one clinical paper, someone gave me a STATA spread sheet, said - go figure it out, it took me about 3 months, and got published.
Basic science - the type that "carrys weight" is what many refer to a "bench top" research. This generally does take longer - in the immediate time scale... the major reason is technique. Its just like surgery, there is a knack to it, you just need to do it and get good at it, and it takes time.
Iwould say big Epidemiology papers take a ton of time... years... Harvard Nurse et al. Basic sciende, may take years. Nobel Prizes are won for progressive work, done in small increments, over years, decades even. Sometimes you can get lucky and get a great paper out in a short ammount of time. That happened to me, and it took me 3 years to do that paper.
If you want to get the basic science, my advise would be to take a year off, get into a fairly big lab, where you can get your own guidance but get your own project.