Rod, are you left handed? DX and pro D start to turn right as you beat them in. They also get longer. Z plastic lasts forever. I have a z xs that is pretty similar to what it was when new (it has been into trees, cactus, and gone swimming).
Also, z and I guess champion, will often come back if you turn them over. Dx and D help you learn more about the disc.
For me, it was financial; I used z because I couldn't afford to replace the lower end plastic. Now that I'm older, I've gone to x and Innova pro plastic. It seems to last a long time, feels better to me, and is longer.
Their are some discs where I like the z version better (flash, xpress, and predator), but for the most part of gone to primarily x. Also, I have 2 z mids because I seem to hit many trees with them. Still, I plan to purchase one in pro d.