I was mentioning the use of the sensor output directly because: it is the same signal used by your scale, only 2 differential signals to interface with and interpret. Simple: just one instrumentation amp which has a very high input impedance, both you current scale and AVR can read the same thing.
If you go for an Atmega32u4 (VinciDuino, Leonardo), you don't even need the differential amp as it is already in the chip. Simple connect both leads the the board.