So it is basically a object tracker?
My Idea for you is to mount a seprate set of "eyes" on a servo and that can track the object while the body lines up with the "eyes". Infrared has a very wide viewing angle and is cheap, and Ultrasonic is more expenisive, has a narrow viewing angle, and takes up more power than IR. Chose wisely young grasshoppa!
The Robot head and robot body configuration is called "horse and rider". Google it!
A walking robot can be slow, costly, and hard to configure. IF your walking robot had,say 4 legs, it would need 8 servos! 4 to rotate the legs and the other 4 to articulate them. Wheel and caster can be easy to minipulate, but walking can climb over much steeper terrain.
Good luck!