The larger of the two is the best as stated. I doubt the kitten will use the litter box while the vehicle is in motion (too much movement for unsteady kitty legs) but probably will once the vehicle has stopped for a bit. When taking your kitty to your vet you can use the smaller one.
What I would like to add is please fit the kitten with a small kitten harness before leaving the breeders'. That way when you stop to let the kitten out for a stretch (inside the vehicle) you can quickly snap a lead on the harness. In case the door is opened inadvertently when kitten is out you do not need to worry about kitten darting out. They may be tiny but they can and do panic when in unfamiliar territory. When they panic they tend to want to run. JMO
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!