The difference in screw pitch will mean very little performance difference. The difference in efficiency, however, will be a lot better.

Now your not going to like this next bit but it's in your interest it's said.!
The Z axis is THE most important part of the machine so whether Acme or Ballscrew won't make a jot of difference because of the weak nature of this Z-axis design. Those V bearings are simply not good enough for Z axis and this design will be massive weak link. It would be wise choice if you changed the design and invested in some profiled linear bearings.