It doesn't make a bit of difference. The solution to your problem is to promise to wash spouse's car every Saturday and have her park outside. You can run it through H2GO for about $6. And if the inside is really bad, they have a machine to do the floor mats, no charge and there is a vacuum at every drying station.
Nice idea, but a no-go. Car lives in the garage. Tools get wheeled outside and used there. She had the deal notarized.
The question is still open:
1. Can one system do both jobs (gadgets and stacked boards) well over the long term? I don't want to hear a stack of heavy boards come crashing down on my spouse's car in the middle of the night, okay?
2. Which is stronger, French cleats or slats?
3. I have a blank slate, and want to make the right choice only once. Which would you choose?
a. French cleats
c. French cleats for gadgets but purpose-built shelf supports for the stickered boards.
d. Slats for gadgets but purpose-built shelf supports for the stickered boards.