If you use UW-imap you get POP3 and IMAPv4. If you set it up right you can do POP/IMAP over SSL
In terms of speed, people say IMAP is faster because POPing downloads all your mail( or you can leave copies on the server). IMAP reads in the headers of the the emails so there is less to download.
If you want to use Squirrelmail I believe you need to have an IMAP server running.
I personally have a RH7.2,sendmail,Apache1.3 w/ SSL support,UW-IMAP(s), and Squirrelmail setup. It works great!
I was thinking of setting up a whole new box on RH7.3 with all updated packages. If your system is similar, what do you say we work on it together and doc everything we do and make a PET?