All the versions of the PET microSD can be used with the original 2001, but 2001's with BASIC 1 need to be upgraded to BASIC 2 or BASIC 4, you can use one of my ROM/RAM boards to have a configurable BASIC 1/2/4 ROM and 8/16 or 32K RAM upgrade.
Which one do you choose? Well choose 1 if you want to move it around between PETs or easily remove it, or don't want to open the PET case. Choose 2 or 3 if you are short of space, or prefer less wires around the back. Choose 2 if you need access to the SD card, Choose 3 if you don't plan to change the card often, or are happy to open the PET as required. Not all PETs support all options, so see the table of which PETs support which boards, which also has buy it now links.