Currently, Mail preferences has the option to switch between "Compose messages in Plain text" and "Compose messages in HTML".
While this is nice, it would be even better to have the same switch available in the mail composer to switch on the fly, based on what is needed (for instance, a user who typically sends Plain text mails may want to switch to HTML to point out something particular in formatting once in a while, or a user who usually sends HTML mail may want to switch to Plain text when corresponding with someone who complains about HTML mail).
The config entry from preferences should just be the default (and while at it, it may make sense to add the default option of "reply in whatever format the message I'm replying to" is -- reply in HTML to people who sent an HTML message while replying to plain text messages as plain text by default).