What exactly is the between 1-bit SDIO vs SPI? Does one have advantages/disadvantages over the other?
The SD protocol uses separate lines for commands and data, whereas SPI uses a single wire pair for both. So, in some situations, 1-bit SDIO may be a little faster than SPI, as it's possible to send commands concurrently with data.
On the other hand, the SPI protocol is publicly documented (with lots of readily available implementations), whereas the SD protocol is not. Combined with the fact that most microcontrollers support SPI in hardware, but only a few natively support SD, this makes SPI a much better choice for hobbyists.