Here's a really interesting comparison of all of the 2014 Naish twin tip kite boards.
The first image shows the differences in rocker between the boards. Boards with more rocker will generally have smoother landings and a smoother ride but also make it harder to go upwind and require you to ride with more power in your kite.
The second image shows the differences in outline. Outline plays a big role in the way a board rides and feels. If a board has more outline curve the tail block sits deeper in the water giving your more control, especially in choppy conditions.
Flex also plays a big role in the feel of a board. Boards with more flex tend to be smoother in choppy conditions and easier to ride but can feel "sticky" when you're trying to load and pop. High flex boards also make it harder to maintain control when coming in fast from big jumps or tricks.