Interior construction KS/brick/metal stud frame

Well, the contract states brick or KS for the interior walls, so nothing in the statics would have to be changed because brick works. The only question remains whether brick is good for interior walls or if the disadvantages are really that serious. Unfortunately, I have never had any contact with brick walls.

Brick is not bad now, but KS is simply better

That is of course annoying and would rather sound like an offer for a price reduction from the person responsible to you. It depends on your needs regarding sound between the rooms and also who and how many people live inside. We currently live in a newly renovated listed building and sometimes brick and then plasterboard or OSB were used respectively with a stud frame. For us that makes no difference at all or I prefer OSB because I can easily screw something into it. With brick it can be more complicated with dowels etc., but as I said, it has to fit your circumstances. What I also like about lightweight construction is that such walls can be changed more easily if necessary than solid masonry; we have done that quite often - basically, however, I wouldn’t let myself be talked into anything there either or if at all, then I would want to gain an "advantage" from it.