When should you use Qnet, and when TCP/IP or some other protocol? It all depends on what machines you need to connect.
Qnet is intended for a network of trusted machines that are all running QNX Neutrino and that all use the same endian-ness. It lets these machines share all their resources with little overhead. Using Qnet, you can use the QNX Neutrino utilities (cp, mv, and so on) to manipulate files anywhere on the Qnet network as if they were on your machine.
Because it's meant for a group of trusted machines (such as you'd find in an embedded system), Qnet doesn't do any authentication of requests. Files are protected by the normal permissions that apply to users and groups (see "File ownership and permissions" in Working with Files), although you can use Qnet's maproot and mapany options to control—in a limited way—what others users can do on your machine. Qnet isn't connectionless like NFS; network errors are reported back to the client process.
TCP/IP is intended for more loosely connected machines that can run different operating systems. TCP/IP does authentication to control access to a machine; it's useful for connecting machines that you don't necessarily trust. It's used as the base for specialized protocols such as FTP and Telnet, and can provide high throughput for data streaming. For more information, see the TCP/IP Networking chapter in this guide.
NFS was designed for filesystem operations between all hosts, all endians, and is widely supported. It's a connectionless protocol; the server can shut down and be restarted, and the client resumes automatically. It also uses authentication and controls directory access. For more information, see "NFS filesystem" in Working with Filesystems.