I'm from CoolBoard, saw this post and thought I could help get down to the nitty gritty... You've hit on the main difference - what does the board balance on, and this effects how it can move. By balancing on the board your body will be training itself to handle this movement. Generally, the more movement - the higher degreee of balance and fitness training.
Indo balance on a straight roller, and so rock and roll (or slide) from foot to foot. They also do a cushion/disc which gives toe to heel rock, but removes all roll/slide movement. So you choose between rock and slide in one direction or rock in all directions but no slide. Feedback is that Indo is quite easy to get the hang of which leaves users trying tricks.
Vew-do can come with a veriaty of bases, their conical rollers are the most common and closest to snowboarding, although far more like skateboarding. These give an interesting combination of foot to foot rock and slide at the same time offering rock in the toe to heel direction due to the conical roller. Vew-Dos are more challenging than Indo, replicate skateboarding the closest and there are many tricks you can do, similar to skate tricks.
CoolBoard is balanced on a ball - this gives rock and slide in all directions removing the movement limitations from other boards. Although now seen as a fitness tool, I developed it to help me learn to surf as the unrestricted movement was the closest I could get to surfing. Since then many surfers and snowboarders agree. Some say that balancing on a CoolBOard is actually harder than on a snowboard... but I don't know because I've never snowboarded. What our customers have said is that it works, especially the slide between toe and heel, which can be the tricky bit to get when learning how to snowboard. Once you can balance on the ball its possible to slide the board over the ball until the ball is under your back foot - this is perfect training for powder (weight back, freely floating movement in all directions). By then taking the ball under the other foot you can practice switch. We also offer a disc, but instead of adding movement, we use it to limit movement for those new to balance training. The downside of CoolBOard is that tricks are very difficult, but then balancing on the ball is way fun enough!
Trying my best to be impartial so I'll stop there (please someone let me know if I've been 'selly', I'm trying to help not sell!)... Any balance board use will undoubtedly help your snowboarding in some way... so I just think it is important for people to actually understand the difference between the products to make a decision on what they will get as opposed to how we market. Think about how your snowboard moves, what you want to improve and make your choice.
Any questions in the field of balance training, fitness or rehab, or just wanging it about on a board, please let me know...
Keep it steady,