HTML or Plain Text
It is always better to send and receive email in Plain Text format. With HTML emails it is easier to get tricked into going to a fake site (phishing) simply because you cannot see the url. With plain text the url is obvious.
Spammers use HTML images to discover if their spam is being read - the minute you click that spam email you have told the spammer to send you lots more.
HTML email takes a lot longer to write and has no more effect on the recipient. In fact if the recipient is smart they will be only viewing emails in plain text so your effort is wasted anyway.
Email clients are easily configured to send and receive in plain text. If you run a business it's essential that all staff are sending and receiving in plain text to minimise spam and phishing and the very real threat that all your office PC's will be hacked.