Will OCS or ROTC be better for me?

OCS selection is quiet competitive. It would be more of a sure thing to contract with ROTC. Also, with ROTC you usually get very good grades and it helps pump up your GPA. And frankly, ROTC is much less intense than OCS.

ROTC may be your easiest/quickest route to a commission if thats what you mean by "better." If you decide to do OCS, you can try to get either the State, Federal or Accelerated program. I believe OCS is the "harder" option unless you go the Federal route.

There are a number of factors, that when combined, will make you competitive for the 72D selection board. Note: you can only commission 72D in two ways: as a biology or environmental major attending Westpoint, or by having experience/credentials, and applying through a Direct Commission package (via an Army Medical Recruiter). They recruit about 20 per year, with 10 coming from Westpoint, and the rest through Direct Commission selection boards. The DC route is highly competitive, with about 150 applicants per year for 9-11 positions (on average). The people selected usually have direct civilian or military environmental and/or public health experience, and a qualifying degree. People with a qualifying degree and no experience are the least competitive. Note: I was selected in October 2018, and just went through the process.
Academics: Having a BS/BA (or higher) in an environmental discipline is the cornerstone to being competitive. Personally, I earned a BS in Environmental Health. A key thing to note (this comes directly from the 72D Consultant): 45 semester hours must be a combination of science and math courses (any combination). Course titles like geology, botany, biology, epidemiology, environmental science, etc., all count towards adding up to 45.
Certification/Licenses: Personally, I had already earned a state-level license as a Registered Environmental Health Specialist, and had the Dept. of Defense Pest Management Applicators License (ties into disease vector control). Any kind of public health, environmental, or related professional certification increases your competitiveness.
Letters of Recommendation: You must have at least 3, but you can have up to 5. At least 3 of my letters were from Lt. Col’s or higher, and 2 were from my academic program. Definitely aim for 5 letters from people that matter. The "higher up" your letters are, the better. For example, a letter from your environmental science department head and/or program director is stronger than a regular associate professor on staff.
Military/Civilian Experience: In general, being prior service is a huge plus. Likewise, having completed service schools (NCO Academy, etc), are a plus. If you already work in military health, that will give you a slight edge, especially if your MOS is related to 68S or another preventive medicine or veterinary corps MOS. Likewise, civilian experience in an environmental or public health field makes you highly competitive.
Volunteer Experience: For 4 years, I volunteered as a merit badge instructor for the Boy Scouts, teaching the Environmental Science Badge, and the Public Health Badge. Doing this in your free time really communicates to the selection board that you embody the 72D MOS.
To conclude, all of these factors combined is what got me selected, and definitely made competing much easier. Good luck.