For Zoho Projects: how to choose between books or invoice?

For Zoho Projects: how to choose between books or invoice?

I am starting to develop our company's project management in zoho projects.  I am looking into invoicing and budgeting. The guide says I can choose to integrate either zoho books or invoice, but the choice is irreversible once I make it.  

I have poked around both apps, and looked through the forums and I have not yet found a comparison of why I would choose one over the other.  From what I could tell, books includes all of the features of invoice, and much more.  But is there a reason why I would pick invoice and not books then?  Could people offer me some insight?

Also, they are both free with my zoho projects subscription right? 

Thanks!

Sam