Hello,

Both reporting tool are useful for generating report.

Thats depend on your choice and interest. If you have nice power on html, css designing than you can go with qweb reporting.

While pentaho provide drag and drop functionality for designing report. Easy connection with database. Allow to print report in csv, excel, pdf etc formate.You can view report preview at designing time.

Hello,

Probably,Qweb is good option for usage if you've sound knowledge of HTML and Styling. and it is very simple to understand. just go through the documents from odoo.

Here , some of the links provided which you can use...

https://www.odoo.com/documentation/8.0/reference/qweb.html

https://www.odoo.com/documentation/8.0/reference/reports.html

While qweb is embedded in Odoo, pentaho need to be installed and linked to Odoo.

Pentaho need a little configuration to suit your needs but you can be ready to deploy in a couple of hours. The learning curve is better than qweb and you'll have a much powerful instrument to query and represent yours databases. 

Qweb is fastest, but need a html xml coding skill and you will work without a WYSIWYG interface.