Bulk email sent in HTML format generates higher response rates than bulk email sent in text format - unless the recipient can't view, or doesn't want, HTML-formatted emails.
When given the option, approximately 15% of your subscribers will request to receive email from you in text format. That rate might be higher if you have a lot of subscribers who register with their work email address.
When in doubt, use email software that can "sniff" whether the recipient can accept HTML-formatted emails. If so, deliver compatible HTML to them.