As ++choroba correctly points out, PM <code> is not HTML <code>. See "Markup in the Monastery".

The PM <code> tag provides some conveniences. It automatically handles certain special characters; for instance, you can paste code with $x < $y without having to manually change that to $x &lt; $y. It also adds the "download" link for blocks of code.

The <code> and <c> are interchangeable. I usually use the former for blocks and the latter for inline: that's just a personal preference.

With <pre> and <tt>, you will need to manually edit special characters; accordingly, I try to keep these as small as possible. You also don't get the "download" link.