There were two types of V5: the regular V5 and the V5 Celebrity (which was sold as a V5.5 in some parts of the world.) They're basically the same, just different built-in game and you can raise different characters.
The Tamagotchi Music Star is the new one. It's just recently been released in the USA, and isn't available in all stores yet. It will be released in UK next month, and Australia the month after that.
Meanwhile, the TMGC+C (Tamagotchi Plus Color) was released in Japan a few weeks ago. As the name suggests it has a color screen. Currently Bandai is saying this model won't be released outside of Japan.