Advice On Breather Membranes

Mostly clients can be divided into the following categories;

A: Couldnt give a toss about the spec, here’s my money, get it done

and

B: Couldnt we use 18mm instead of 24mm for that piece of concealed ply that nobody will ever see?

So recently I have a B type asking me whether we should use Tegral’s Ventex breather membrane or Tyvek’s equivalent on their counter battened slate roof. Does anyone have a clue as to the merits of either? All I could say was that Intello plus wasnt necessary because the building wasnt ever going to be air-tight since it’s a conversion of a 1960s building.

Any advice appreciated.