From what I understand they all serve the same basic functions (tank cleanup / random snacks). The differences are in size, breeding habits, and breeding speed. I have some dwarf purples that I can barely tell apart from springtails, oranges that aren't breeding very quickly (but hey, they're orange!), and some giant canyons that breed like rabbits and are about the same size.
Definitely get your hands on some Giant Orange Isopods. Awesomely huge (bigger then a TicTac) and fun to see out in the tank. Dwarf White Isos will breed the fastest in tanks and, the striped dwarf grey too. The Orange are quite large. The babies will be consumed allowing the larger mothers to continue breeding. The white and grey are smaller and good hiders. They will give a steady supply of food.