WCF offers more flexibility,reliability over web service as it supports Named pipes, Message Queuing,http, tcp etc. protocols.
Web service only supports http.
WCF can be hosted in IIS, windows activation service, Windows service etc.
Web Service can only be hosted in IIS.
WCF uses system.runtime.serialization for Data Serialization which again provides performance benefits as compared to System.Xml.serialization which web service uses for data serialization.
Also WCF provides more security as compared to Web Service.It has all the power of .net Remoting,Message Queuing etc.