If you're dealing in sensitive or proprietary information, that's good news. It's pretty darn complicated to set up. Well, quite frankly, not much. Although the technical side of things make Slip and PPP quite different, and PPP is a newer protocol purporting to have better compression and transmission speeds, for your purposes both are essentially the same. The real difference, you'll find, is the quality and reliability of programs that implement these protocols.
Quality varies dramatically.
Of course, you needn't dial in directly using your terminal program. Instead, once you're connected with Slip or PPP, you can use a telnet application on your PC to telnet from your computer to Citilink's. PPP negotiates all of the settings that can make the connection unsuccessful, SLIP requires you to know the settings before attempting the connection. If the PPP server supports the Password Authentication Protocol, a connection can be mode without any user scripting. You will be prompted for a username and password if it is not entered.