Glulx is a 32 bit virtual machine, while Z-machine is a 16 bit irtual machine. It has its own set of instructions and its native multimedia (including text output) library is Glk.
In general, you can use both z-machine and glulx-machine to run Inform games, though Glulx would be better for larger games, and specially if they contain multimedia as graphics and sound. Also, spanish authoring system Superglus compiles to Glulx, but not to z-machine.