We ran the same tests and text emails won every time, and it is the same story with many of the marketers I talked to.
There are a variety of reasons, but for me the key drivers are:
1. Text emails are inherently more personal because humans do not send HTML emails 1 to 1
2. It's hard to get user experience right on mobile - even with deep technical resources. Text emails are generally mobile ready and you can consume the content in a few seconds.
HTML emails will always have their place however, especially for branding initiatives, live events, etc.
Darrell Alfonso, I think your key drivers are incredibly insightful. None of us send HTML emails to friends, family or colleagues. And, none of us have time to waste reading complicated (i.e. HTML) emails on our phone. Thanks for sharing!
I've used both regularly, and definitely think things such as event invites or more personalized content is better in a text email, but promotional/marketing/branding emails are great in HTML.
Another benefit of using text email is increase the chances to land in Gmail's primary tab. However, as Juli James and Darrel mention - HTML always have their place and are great for promotional/branding emails. E.g. though a promotional/HTML email is more likely to be received in Gmail Promotions tab, it seems to be an attempt to facilitate advertising in the mindless moments - i.e. Gmail could be expecting users to actually have the intent of going through the promotions tab, and browse through promotions - with the mindset to find promotions instead of finding promotion in the work area of primary tab.