According to the New Sensor data sheet on the KT150, the heater current is not less than 1.75A and not more than 2.0A and for the KT 120 it's not less than 1.7A and not more than 1.95A. So, they're pretty close in that regard.
There may be other characteristics, physical, mechanical or electrical that may preclude using the KT150 in applications that are designed for the KT120. Physical size is an obvious one.
There may be other characteristics, physical, mechanical or electrical that may preclude using the KT150 in applications that are designed for the KT120. Physical size is an obvious one.