I looked it up on Makita's site, once I found their comparison feature. Both are pretty big upgrades, but here's the "tale of the tape":
XDT14Z (delta to XDT13Z)
torque: 1,550 in/lbs (+50)
weight: 2.9 lbs (-0.4 lbs)
length: 4.625" (-.375")
max RPM: 3600 (+200)
max IPM: 3800 (+200)
modes: 3 (+3)...