Well, I'm certainly no expert, but I can only think of two reasons why you would switch: (1) there is something specific about the Z72 or Tungstens that you would prefer over your Z71, or (2) you just want to try something new. If your Z71 is over a year old, you shouldn't expect to make out well financially on the trade, so why not stick with the Z71?
With that being said, I think all (or at least most) of the new PDA's will have great screens. I just got an E and the screen is wonderful, but I selected it more for its size and price than the screen (again, because most models have good screens these days).
-------------------- The first rule of holes: when you find yourself in one, stop digging.