Whole emails is the one you want.
I would create the HTML email, and then create another email that is text only.
Then do whole emails test.
Thank you, Darrell!
As Darrell mentioned, a 'whole email' test is the type of test you should create. We've done this test in the past, and although it will depend on the segment you are sending this email to, we realised that text emails tend to perform way better than visual emails as they seem to be much more personal.
Hope this helps you,