Romanian deadlifts and good mornings are both hinge patterns, however there are some major differences. Such as the lever action in relation to where the load is placed. A true good morning is a posteriorly top loaded hip hinge. The difference being the loading vector and range of motion.
The good morning is what is called a class three lever (Romanian dead-lifts are a class one lever).
- The load is at one end, in this case on your upper back.
- The force is generated from the musculature of the hips, with the bend
(or fulcrum) of the movement bearing down from the pelvis, through
the feet, and into the floor.
Essentially in a good morning, the entire length of the body is acting as a lever arm with the load being distributed throughout your entire posterior chain. That's a lot of muscle worked. This is an important concept to grasp. The good morning is not simply a "lower back exercise". Done properly, the good morning works the entire length of the erector spinae.
I would do both as your PT has recommended.
Put very simply - yes the motion is very similar and they're both great exercises. The same "major" muscles are activated but at a different times and for different durations. You're teaching your muscles to activate correctly. It helps with balance, stability and improves overall movement quality.