Just a slight correction... panamax means it's the largest thing that can fit through the canal, post-panamaxs don't fit.
The better answer is, it depends. The post panamax container ships are faster, but draw more water, so the Asian loading ports as well as the North American discharge ports are limited.
Some steamship companies run round the world voyages with smaller ships, some break-over and reload in various places.
It's generally almost always cheaper to move the container by sea than by land, especially if it stays with the same shipping company.
But it all depends on what rates are negotiated between the shipper and the carrier. And how many containers and how regular the requirement is, as well as how fast customs clearnace can be arranged, etc.
Canal space is also sometimes at a premium, that's why the railroad was rebuilt as well as the crude pipeline there. The larger carriers book passage times well in advance and aren't delayed very much, but like the MN part of the NE corridor, if they lose their slot, they can be pushed back pretty far.
Sorry for the ramble, but this is an everyday thing for me.