MT3333? Yea that's an improvement over the UART interface where it just sends out the data no matter if the other side is listening or sleeping. With the SPI interface you get an interrupt and can read the data at your own pace.

But I2C would do too if they would offer a proper register interface as opposed to always expecting you to parse those verbose NMEA messages.

Oh and only generating an interrupt if the fix is valid would be nice! But they just shoehorned the SPI interface onto it, exposing exactly the same data as they would over UART. So if you have to wake up, parse the message and only then will you find out if you have a valid fix or can go back to sleep and try again.

Afaik they dropped the SPI mode in the newer chips though.