WebPlus X6: Joomla v Serif WebPlus

Hi
For those who have ample knowledge this question may appear odd, but my question is this:

I have only ever used serif webplus products and have been happy doing so, even making websites for others and being paid.
I now have an enquiry which may lead me to helping them with ideas and changes to their existing site which I am told is Joomla..
Obviously I have seen joomla mentioned but how much does it differ from serif webplus which I consider to be a drag and drop build program ?

Hope my question makes sense and someone can throw somw light onto this.

Regards

Stewart