Does either the Nikon Z vs Canon R UI offer quick switching for storage folders?

The Canon 1-Series bodies have a few dedicated buttons and even a small monochrome LCD on the back of the camera for file/folder settings. I'm not aware that you can do that on any other Canon body using a direct button. However, many Canon EOS cameras have a section named "My menu" where you can place shortcuts to many menu items on a single menu tab. You can also create more than one "My Menu" tab on some Canon cameras.

For the EOS R, how to set up the "My Menu" tab is covered on pages 565 and following of the EOS R Advanced User Guide You can download the pdf at the link and research many of the capabilities of the camera and how one might customize it.

One of the options that can be placed on the My Menu tab(s) is the 'Record func+card/folder sel.' (Record function plus card and folder selection) menu.

One of the three sub-options in that menu is folder selection. It allows you to create additional folders or to select among existing folders.

I generally have the menu centered on the "My Menu" tab so that when I press the "Menu" button I see the settings I have added there that I most frequently use. One press of the 'Set' button would then place you in the folder menu.

Even if you last used another menu tab, when you press the 'Menu' button and the menu comes up with the last used tab visible, you can skip around fairly easily by pressing a single button that jumps between the various groups of menu items: Shooting (Red), AF (purple), Play (Blue), Setup (Yellow), Custom (Orange), and "My Menu" (Green). You then use the main dial to move between tabs in each group. It's pretty intuitive once you use it for a bit.

You can also set up the entire menu so that any time you press the 'Menu' button, it always opens on the "My Menu" tab, rather than on the last tab you used, which is the default setting.

I can't speak for Nikon, so I'll let a Nikon user answer that part of your question. I doubt there's anyone here who uses both Canon and Nikon enough to be able to fully answer both sides of the question.

For Nikon Z, the fastest way seems to be:

1. Add "Storage folder" to "My menu" and move it to the top
2. Assign "Access top item in MY MENU" to an fn button

You then get the "Storage folder" menu with one button press. You still have to navigate the menu to select a folder from the list (down, left, select, ok).

I did not find a way to directly assign a specific folder to a button.