MM6/MM2 differences when compared to MM5/MM1:  
- works in countries where 2G is switched off (USA, etc).

- consumes 30% more energy, so battery lifetime is 3-4 months shorter. The reason for that is the below improvements:

1. It provides more accurate GSM approximate location, which helps a lot when GPS signal is not available (underground garages, GPS signal is jammed and so on);

2. Better chances to get a GSM connection. MM6 covers both, 2G and 3G networks - if the 3G signal is low, it will search for 2G service provider and connect to it (and vice versa).

See list of MM1/MM5 supported countries here and more details on MM2/MM6 here.