You can send both formats, and in fact, if you send text/html, you should send a text/plain copy as well. As one of the remaining mutt users out there, nothing annoys me more than html mail. I don't read it.
I much prefer plain text, but this really depends on your target audience.<p>Based on personal experience I believe those who don't live and breathe this stuff 24/7 prefer a little window dressing.<p>If it is styled, I recommend minimizing the amount of scrolling required to reach your text. (For example, I don't see much value in a large header mit logo.)<p>But test, and ask the same question of your prospective readers, if possible.