Well here is a debate that has gone on for years and here is what I believe.  Text based emails will ALWAYS get more deliver-ability than HTML emails.  I use both but I also believe in a number of cases the branding ability with HTML and open and click tracking available in HTML emails there is a strong argument to use this too.

So test your results and see what your database ersponds better to.

Here is a link to a really cool tool to format text based emails so they look better and make it easy to format.

http://www.formatbluesoftware.com/textformatterplus.htm