Your business,your client,but!
Sometimes their are certain material choices You have to take control of.
In our cabinetry business we have these material choices come up on a daily basis.
When I sit down with my client and go over final specifications I go over with them the substrates we will be using on the job.
They pick the material species I pick the core. 99% of the time.
I get questioned often. Why don't you use particle board we know it is cheaper?
I tell them why we do not use it and what we are using and why it is so much better.
Back to your issues. This work reflects on you and your business. When this company has one of there important clients sitting around that conference table and he/she says that's a really nice picture but who the heck mounted it and why is it all warped.
The guy that ordered from you is going to spit out your name and say "I have no idea why he mounted it like that " giving you all the bad credit.
I would give them 2 prices,one for your metal and the second for gator.
If he asks about foam core you tell him it is not used for anything over 16x20 as we have had too many issues with larger sizes and cannot manufacture something we cannot warranty.
Have no idea where the cost is but a 30x40 print and mount is not inexpensive.
1/2" gator is not expensive and cannot imagine the difference being an issue.
We use 1/4 and 1/2" gatorboard all the time with no problems.
Bottom line i would think you would want to make the choices for materials that are so critical to the integrity of a project like this.
Now for the apology part as this wasn't meant to be business ethics 101.
And here all you asked for was white or black.
Sorry the devil made me do it.
It's just that with 30 odd years of this kind of experience I would want to make the choices I know I can stand behind,my/your names at stake.