It really depends on whom receives the emails and what email client the recipient is using. Because HTML is a combination of both simple text and HTML codes, there’s a probability that the content will not appear accordingly to all recipients or it could be even rejected by the email client. However, there is an upside and downside for each one of the options.
If you want to send your emails in plain text, you cannot add other elements such as graphs, but all email clients will accept them. If you want to add rich content to your email template, such as graphs, colors and different fonts, you’ll need to switch to the HTML mode. But the downside of it will be messages that might be rejected from some email clients.
In conclusion, it’s up to you to choose the best option that suits your needs best.