Well, they both look like some "namespacing" mechanism, in that you can enable the next level of key bindings by pressing a certain key combo. But multi-level keymaps will always reset to the root keymap immediately after a command is triggered or an undefined key is pressed, while transient keymaps will stick around until you pop it from the stack.
Yeah, seriously, transient keymaps stick around. You should know by now how bad I am at naming things.