Both are good questions, and they are two questions, not one.
My simple and I hope memorable advice is:
Train Broadly, Write Specifically.
When employers or companies look for medical writers, they typically want someone for a specific job, or a specific type of medical writing. They like to hire and work with specialists - people who know the particular field, who are familiar with the literature and practice around specific ailments, populations, pharmaceuticals, or devices. Everyone wants someone who knows what they are talking about, in order to get the most bang for the buck out of an author.
So, when you are presenting yourself for work, it's best to be the specialist who really knows what they are talking about.
Now, of course you can have specialties in a number of different areas. It's good to do that. That way you can have broader appeal to potential employers or buyers of your content. But they are only interested in the specialty relevant to THEM. Present yourself through your payer's eyes. Be the specialist THEY want.
Now, with regard to training, think about training broadly.
There are about 7 different kinds of medical writing:
Consumer Health Writing
Abstracts and Posters
Biostatistics and Epidemiology
Scientific Journal Writing
Continuing Medical Education (CME) Writing
You can be well known in one or two kinds. But you should train broadly.
That means Abstract and Poster Writing, and Biostatistics and Epidemiology skills are broadly applicable to Consumer Health Writing, Scientific Journal Writing, CME Writing, and Regulatory Writing.
And Scientific Journal Writing, Regulatory Writing and CME Writing require similar discipline in gathering evidence, performing analysis, drawing conclusions, and making recommendations.
So, when you think of getting trained, train broadly. Consider packages of Medical Writing Training.
When you think of getting work, present yourself as the specialist to your potential payer.
Remember this simple advice: Train Broadly. Write Specifically
Jonathan Marx, MBA has been a medical writer since 2001, and owns a company which trains medical writers world wide. http://www.medicalwritingtraining.com. He also manages the annual Medical Writing Festival, an annual online event featuring 10+ hours of online seminars on medical writing and the business of medical writing. He also manages an online network of medical writers from around the world, at medicalwritingtraining.com