The best sd module is the custom design uploaded here, which uses the same schematics as the microSD module but offers a full-size sd slot.
It uses the parts of the microSD module and this sd slot (MMC/SD long variant).
The old sd module works too but there were concerns about the missing voltage level converter chip, so I designed the custom pcb.
You can also use the microSD module, however on almost all cart reader pcb revisions but the currently unreleased design you need to desolder the AMS1117 and bridge its input/output pads on the module's pcb.
I'm still waiting for some Arduinos I initially ordered in February and reordered in April, so that's why the project is a little bit behind schedule right now.