The Leads App is an application that is particularly tailored for the CRM community. It has features like Tasks, Events and Notes and having the option to scan both sides of the card which the Card Scanner lacks. The Zoho development road map for leads is to include all features that a person at a conference would require to generate a lead. A sales person at a conference can scan a visitors badge or business card and add him to the CRM also he will have option to send custom email's using Zoho CRM's email templates for the contact. Leads will ideally become a complete Event Management and Lead generation solution.
Whereas, Card Scanner App is a general purpose business card scanning app. It is tailored towards general audience who wants to reduce the manual work of digitizing business card's. The road map for Card Scanner is to support sync options with Zoho apps like Zoho Contacts Manager, Zoho Books, etc and Other Non-Zoho services like Google Contacts.