I just had to comment on the latest post at the Aweber blog. The question is an important one and a very difficult one to answer. The post at Aweber is very good and you will probably learn a lot from it, I recommend that you read it. So, to my comments.
I have been using both plain text and HTML, and I actually do prefer plain text myself. You can do a lot of things with HTML, like add colors and pictures, and this might catch your readers attention more than the plain “boring” text. But with HTML it´s easier (at least I think so) to get caught by spamfilters. And it´s easier to distract your readers as well.
I think that what you prefer and what you should be using, depends on how good you are with HTML or if you have paid a webdesigner to make you a template. If the template you are using is clean, colorful and without any fancy effects, then HTML will probably be a better choice.
As I said, it´s a difficult question and I see that some professional marketers prefer plain text while others prefer HTML. No given answer to this one, but again, you might learn something if you read the post at Aweber.