The Aria Line M and the Aria Line are relatively similar products. The main difference lies in the cables included. The Aria Line only includes one type of cable: a retractable 3.5mm AUX coil cable. The Aria Line M has 2 cables: a boom mic cable optimized for calls, as well as a regular 3.5mm AUX audio cable for TV.
This makes the Aria Line M the more versatile product, as it comes with a mic which allows you to make calls with your PC or phone.
Other than that, the products are quite similar in regard to sound quality and clarity.