Hi Amin,
it depends on what you call a report and what your users prefer.
If a report for you is a query, then using either Bex Analyzer or Analysis for office would be an easy way because then you only need to provide the queries.
In portal you need to specially provide the queries to the portal - it´s an additional step in development.
If there are special needs from the users for formatting, layout and so on, you need to provide either workbooks or web templates.
in any case you need to train users in how to work with the frontend, it makes not very much difference whether the frontend is excel or web, the users will need some basic training.
regards
Cornelia