my two cents.
The temper of a given piece of leather is affected by numerous variables, including the Type of cow, tanning processes/tannery, and where it is taken from on the hide (i.e., belly or back), and construction techniques are just a partial list of the possibilities. So to make an absolute statement on which is stiffer is silly at best. Given that you want a guaranteed optimal outcome, you will have to do some real-life testing to eliminate as many of those variables as possible. Use specific parts of the hide, specific tannery's, and be very precise with your construction, including the addition of liquors/oils during the finishing stages. Some folks use a thick outer layer with a thin liner, some use two equal-thickness pieces because they can get them from the same hide, thinking it makes construction costs less or easier in some way. Whether one method is better is again debatable and susceptible to the variables. Bianchi probably can handle the cost of more waste than you at the beginning, so they can get refined and consistent results. we, as small batch folks, need to be much more selective to compete.
Contact cement can be diluted and made thinner with acetone. Bianchi probably figured out it goes farther and dries faster that way.
Good luck to you. I can't wait to see your stuff.