The primary advantage of a PPP or SLIP account over a shell account is that the software being used to access the internet can now run in the user's computer instead of the provider's Unix system. This means it can make use of features present in the user's operating system, such as graphics, windowing, sound, multi-tasking, etc. While PPP and SLIP accounts can be more difficult to install and configure, the added capabilities and ease of use can be well worth the effort
SLIP is an older method, easier to empliment in software, but more difficult to configure and use. When header compression is used, it is often called CSLIP. To establish a SLIP connection, both points must have already authenticated each other, and agreed whether compression will be used, and what IP addresses will be used at each end.
PPP is newer, and supports methods of negotiating much of the needed information after the connection is established. PPP can authenticate the remote user via the Password Authorization Protocol (PAP), elimating the need for a "login script". PPP can negotiate what IP address and netmask each end will use, thereby simplifying configuration.