People who shoot delicate stuff, stuff that will move if disturbed (insects), or those who just want more room to work (usually with lighting) will choose a longer focal length for macro so the camera can be further from the subject. The wider one is, well, wider and you'll have to be closer, or be able to fit more in at the same distance. I never work with macro lenses, but I'm assuming the dof won't be as selective with the wider one (although it's still gonna be rather selective since it's macro) at the same focus distance.
Reverser rings are a bit cheaper if you decide to go that route, and if you're gonna get diopters don't get really crappy ones, because they make the image gross. I had to use some in photo class once and the images weren't too sharp.