You could make a decision to use one or the other based on your subscriber base, the message being sent or your personal preferences, but the best thing to do is offer your subscribers both options. Naturally, this means more work for you, formatting the same information two ways, but it also means having a subscriber group whose members know you consider their preferences. Whether you format for text, HTML or both, develop a format that's easy to read. Tips for Text Separate newsletter sections clearly. Blank lines create valuable white space, but too much can be boring.
Line length is another factor that can upset careful formatting.
Standard line lengths are around 76 characters. Not all e-mail clients will format your message correctly if you go wider than this. Besides, individual user e-mail settings will affect formatting in unpredictable ways. Set line length by typing the numerals 0 through 9 until you've reached the desired length. Or set the character wrap to between 60 and 65 on your e-mail client.