Medical schools are going to give you an education but what will you give to the medical school in return?  Think of previous jobs (i.e. full time EMT, firefighter, etc). cultural background (i.e. grew up most of life in Latin America), economic hardships (i.e. grew up in impoverished town in Central California Valley), unique skills (i.e. concert pianist, etc.), or unique college experiences (i.e. started a health outreach clinic, student body government president, NCAA swimmer) that could help the incoming class.

Previous Jobs:  Were you a firefighter, full time EMT, healthcare management consultant, etc. ?  These previous jobs add unique dimension to your incoming class.  You would be able to provide a different perspective to the incoming class. For example, in health care policy debates, you can provide the point of view of a the insurance company if you've had previous job experience as a healthcare management consultant.

Cultural Background: Growing up in a foreign country would give you an interesting background that would contribute to the incoming medical class.  If you grew up in the United States, you could still have been very involved in your culture by participating in a cultural dance group back in undergrad, etc. 

Economic Hardships: If you are talking about economic hardships, talk about the challenges you overcame and how these challenges affected your decision to enter medicine, such as the lack of accessible health care in your area.  Don't make this a sob story about you.  Don't try to elicit sympathy from the admissions committe. 

Unique skills:  I'm amazed how many of my classmates are extremely good musicians (some even perform at the university symphony level) or competed at collegiate level swimming.  Think of what unique skill sets you have that would contribute to your class.  

We don't want just smart medical students, we want smart medical students that have additional skills or backgrounds that would enrich the experiences for their classmates and future colleagues.

In my experience, it really doesn't matter. But if I were to choose one method, I would send an email. For example, if you are half way around the world on a trip and found that your publication was published, then you may just want to send an email to update the school on the publication. Many schools will print out your email and append it to your physical file. Some schools have gone completely digital with applications, so attaching the email would be easier than having to scan and attach a physical letter.