AirParrot 2 is an application that must be installed to the computer that shares its screen. AirParrot is linked to a single computer, and can be used to connect with an unlimited number of receivers.
Ditto is a service, it can be installed, or it can be downloaded as a single use file that does not permanently install to the computer. Ditto is linked to a specific AirPlay or Cast receiver (such as Apple TV, Chromecast, Reflector 2, etc), and can be used with an unlimited number of computers sharing to that receiver. Instead of needing an AirParrot license for each user, one Ditto subscription can cover as many users as needed for a specific receiver.
Ditto connects directly to your receiver. Once a room has been configured, there is no need to discover or select the receiver during the connection process.