I've tried each of them for about half-a-day. I've ended up using froyo because I wanted my gmail calendar to sync with my PE.

With K9 I could only get one email account to work at a time (multiple accounts may be possible -- I just couldn't get it to work in the time I spent.) It shows the subject line of new messages in the notification bar at the top of the home screen -- a nice touch. It uses color as it displays queued messages.

Froyo manages multiple accounts and creates a combined inbox. It uses lovely shades of gray to distinguish email sources -- no, I lie: It uses low-intensity colors to distinguish sources and shades of gray to distinguish read from unread messages. You will know when a new message has arrived by the appearance of an envelope icon in the notification bar: no subject line is provided, but then your notification area doesn't get cluttered with dozens of message subject lines (if you don't keep up with your incoming mail).

It's probably a personal-preference thing -- unless you want to sync with google calendar, in which case froyo is for you.