What is the difference between Nano 328p-au and 328p-mu? What is the difference between Nano 328p-au and 328p-mu? Does it have to do with the updated bootloader or is it just a different board design? I want to make sure I order the correct one and I could not find a proper answer so far. Thanks.
The AU and MU on the chip just refer to the shape of the chip. AU is TQFP and MU is VQFN.

You can see the full list of suffixes here (select the red "Buy Now" tab for a table).