I know this is an older post but still relevant if you're still out there. We've been testing the same thing and seem to get better results with text emails, however it depends on certain variables such as how engaged/receptive the audience is, or what stage of the lifecycle they're in. I also came across this article this morning: Plain Text vs. HTML Emails: Which Is Better? [New Data]
I plan to do some further testing on plain vs. html in the coming week for a cross-sell campaign. Let me know if you've come across any other data or had specific experiences of plain vs. html since this was originally posted--would love to hear others' experiences. Thanks!