I2C or SPI?

I have a XMEGA xprotolab (http://www.gabotronics.com/development-boards/xmega-xprotolab.htm) and I want the oscillated signal to be translated to the brightness of an LED. Should I make an I2C connection or SPI connection to do this? My plan is to have the Master Arduino receive Data from the slave oscillator, find a way to convert it within ranges from 0-255 (resolution of the LED), then write that value to the LED. would this work, or do I need to take a different approach?

The xprotolab oscilloscope module can communicate via uart (115k2). So the simplest way would be to connect your oscilloscope module to arduino via the serial uart.