I would try out the HTML version. The reason that the big companies you mention send out HTML versions is because they work for them. If eBay figured out that text messages got a better response, all of the email you get from them would be text. These guys test every possible variable and go with what works.

Of course, what works for some doesn't guarantee that it will work for you. So I would test it out on your audience. The next time you send out an email, split the audience file in half and send HTML to one and text to the other and see which one results in the most number of purchases (if that is your objective) and not only the number of clicks.

This only works if your sample size if big enough to make valid comparisons. So if your audience file is small, do the split test for a few emails until your sample is big enough.