AppleShare IP provides more services, and can support more concurrent users than Mac OS 9's File Sharing. If you have a small workgroup, and the only feature of AppleShare IP you use is the ability to connect over TCP/IP, File Sharing over IP may be all you need. If you need a Web server, Mail server or DNS server (through bundled MacDNS), then you would benefit from using AppleShare IP.
File Sharing in Mac OS 9 has the same limitations that File Sharing does. Only ten users can be connected at one time.