You can send both formats, and in fact, if you send text/html, you should send a text/plain copy as well. As one of the remaining mutt users out there, nothing annoys me more than html mail. I don't read it.

I much prefer plain text, but this really depends on your target audience.

Based on personal experience I believe those who don't live and breathe this stuff 24/7 prefer a little window dressing.

If it is styled, I recommend minimizing the amount of scrolling required to reach your text. (For example, I don't see much value in a large header mit logo.)

But test, and ask the same question of your prospective readers, if possible.

This is something you really have to test. You can ask, but people often think one thing but react differently.

Most HTML email I receive also has a text version. I prefer text, but I don't see why you can't do both.

For a BtoC product HTML style I'd say: attractive.... For BtoB, SaaS Product: Plain or HTML but very light.

+1 Plaintext using Markdown syntax.

MIME multipart/alternative type is what you want to use. RFC2346 section 5.1.4.

I prefer creating plain text emails. I prefer reading lightly styled html emails.

+1 Plain text.