The KT-78A is a quality King transponder, and an update of the original KT-78. This transponder is has been solid and dependable, but it is really beginning to show its age.
With 150 watts of transmission power, the KT-78A may operate up to 15,000 feet. If you don't fly higher than that, and don't travel a great distance from ATC radar, this lower-powered transponder may be exactly what you need. Why pay more?