both are built on the same (OpenSTA) code base. Quotium Pro is sort of the commercial version of OpenSTA .. But the development has been forked. So OpenSTA is managed seprately as an open source project and remains free. And Quotium Pro is a commercial product that costs $$ and does not come with source code.
Quotium has added some additional modules and reporting features.
If OpenSTA does what u need it to do, by all means use that one! [img]images/icons/smile.gif[/img]