Is it more respectful to address a professor who is an MD as "Dr." or "Professor" in email?
The answer strongly depends on local culture. For example, for Germany there has been a similar question some time ago (Do german professors like to be called &quot;Herr X&quot;?). To make things worse, the culture might vary even between institutions. 

I learned that in Mexico the "professor" is just the job description, while "Dr." is the valuable academic degree. 

In the US, some professors in my field (medical computer science) are very relaxed regarding titles, others are not (but this is an outsiders view). 

The easiest way is to ask a professor you can trust in your institution. Most will happily explain the common habbits and I assume you'll her some interesting anecdotes ;-). 

In general, the most respectful way to adress a professor is "Prof. Dr. XXX".