Unless you're planning to immediately replace the SSD yourself, I recommend going with the 256G version.

The 64G version uses an eMMC drive, which might as well be a microSD card itself.

That said, I use the 256G NVMe version with a few microSD cards and it's been an amazing experience. Games have slightly longer loading times when played from microSD but that's almost completely irrelevant because of how well the deck handles resume from suspend/sleep.