Mostly the difference is the M loses the fog lights in exchange for more airflow underneath. Sport line has those silver fins and the fog lights. Not sure if M has the black kidney grilles either.
I think both look great, I have a sport so I probably am a little biased to lean towards that but in reality both look the best among the different lines.
Current: 2010 E90 M3 - JBM PP,TP,ES,CW,Beast
Retired: 2012 F30 335i Sedan - Sport Line