Hi BN,

The Harmony M's advantage is in its simplicity of operation. TC Helicon did a great job of making it simple to use because a gigging artist doesn't want to have to waste time fiddling with settings in a live situation.
The audio quality of the harmonies is amazing and sounds nothing like the computerized ones. You can tweak the correction time to superfast so that the effect is a little wierd if you so wish.

The Voicelive 2 has many more features and settings. While it is also can be used in a live situation, it is much larger than the Harmony M. There are many many more parameters for tweaking the audio and also the mix of the resulting harmonies with relation to your original voice. The quality of the harmonies is also better IMHO.

Hope this helps!