What is the best serial communication method between an Arduino and a spectral sensor?
The I2C on this device is slave, not a master. Even though it has its own clock, it is designed to operate as a slave being read by a master.

You will have no issues using the I2C interface - in fact Adafruit uses it for their own software library.

Since there is already a I2C-based library available from the manufacturer I would use that, unless there is a good reason to use the UART instead.