I want to order one of these books to learn how to do the kaliedescope quilts I have been seeing. I have looked at these books and they seem very similar.

Which one would you buy and why?

Stack and Whack, I have it snd its a good one, have made several from it.

I prefer Stack and Whack.

It depends on which look you prefer. I love the OBW's as I have made one with hexagons all together and it looked great, the second one I did, I didn't like the look of all the hex's together so I added triangles around them and it looked more like a Stack and Whack.
The first OBW book shows you both Hexagons and Octagons and the second is the hexagon with the 3d cubes.

OBW is kaleidoscopic - made of 6 equilateral triangle that nest together.
S&W are also kaleidoscopic but set in blocks. The S&W book includes all the pattern pieces.

One Block Wonders ---- one fabric, one shape, one-of-a kind quilts by Maxine Rosenthal

Why? because that's the one I used to make my OBW.
It gives you the instructions from selecting the fabric, to the how-to basics right thru to completion of both the hexagon OBW and the octoagon OBW/SAWs.