I agree with both sides! To me, it depends on the horse.
If she is sensitive, a halter will do the trick, But if she is "heavy" you might be better off with the side pull!
I would also have to ask how much ground work has she had and what has been used on her, as well as are you wanting to ride her with the side pull or are you using it in her ground work?
There are to many questions to answer you honestly.
If you are happy with your trainer and plan on using him for the duration, I would suggest following his instruction.
Hope that helps some!