This is a very broad question, But the main differences are :-
Reportnet is a solely server based product where reports are created and distributed via the web only, where as Crystal is windows client based software.
As with most Cognos tools Reportnet requires the set up of data catalogs with predefined database connections and table joins before reports can be developed. Where as in Crystaldatabase connections and tables are added to Crystal reports by the user .
Cost of ReportNet is thousands compared to Crystals hundreds of £/$ 's
Gary Parker
MIS Data Analyst
Manchester, England