Also, how does CM determine if a recipient should receive HTML or Text? In a previous ESP I used there was an option for subscribers to choose HTML or Text. Is this choice less necessary these days?
We send emails multi-part, meaning both the HTML and plain text are sent in 1 email and the email client itself chooses which to show depending on settings or capabilities.
If you had a large number of plain text subscribers before you can always continue to ask that question on your subscribe form (use a custom field with options for HTML and plain text) and create segments based on that question and then send a separate plain text only campaign to your plain text only segment.
If you're only sending the multi-part message to test your plain text version you'll need an email client capable of switching between versions. You can see some information on email clients and how to test here.
Still not got an idea on how to tag the text version to the HTML email...
What do you mean by 'tag the text version'? Do you mean you want to link from your HTML version to your text version?