The Meets is the most recent line of colour models, and the M!x was its predecessor.
The Meets continues on from the M!x in that it has a genetics system (so, when you marry off your Tamagotchi, its offspring will inherit traits from parents, grandparents, and so on), though it also offers pets for your Tamagotchi, and the ability to raise twins - both twin Tamagotchis and twin pets. It also has Bluetooth and can connect to a mobile application that will allow you to marry your Tamagotchi to those of other users, or to whatever guest character is currently available.
The Tamagotchi Meets is notable for being the first Western release of an English-language colour Tamagotchi, under the name of Tamagotchi On, though it's only available in North America, with no word on whether or not this will change.