Okay, I don't agree that is is only marketing that is making the santoku popular, nor it's aesthetics.
I want to best serve the OP , I'm also not interested in the ego dance between brothers, just to be on the same page emotionally.
I agree that you use the knife how you want, but
it is best to use it in a manner that keeps it's attributes.
I think the santoku is a great tool because it's thinness and the space between the knife edge and handle means it is super at doing slice and dice veges, it is just a great light and easy tool for many kitchen jobs.
A deba with it's fat blade is not good at slice and dice veges, in comparison.
It is better at pressing through soft tissue.
To aid the OP in his decision, I make the point that a nakiri added to his collection would do those tasks admirably, plus he gets a grater, shredder, hammer, tenderizer as well, but the deba isn't going to fill that space.
He light rather go with a santoku but he is a smart guy, he survived living in Perth!
He will pick out the info he trusts the most and make his decision.