Actually Z-wave uses a lower frequency so should have greater range than Zigbee. Stats say 100 feet for Z-wave and 30 feet for Zigbee - line of site, no obstructions but how often does that happen??
Zigbee uses a higher frequency hence the reduced range over Z-wave. The ability to communicate over a greater distance all depends on a lot of factors including transmit watts and receiver sensitivity.
Regarding frequency and range, a good example is with wi-fi networks; a 2.4GHz wi-fi network has greater range but slower data speeds than a wireless 5GHZ wi-fi network so the closer to the access point, the higher the frequency the faster but farther away, you won't be able to connect at all unless you switch to a lower frequency. You can test that with a laptop, tablet or phone. That's wi-fi technology but you get the idea in the case of Z-wave vs Zigbee.... In the case of Zigbee, I believe Zigbee primarily operates at 2.4GHz like most everything else - bluetooth, Wi-Fi but also uses 915MHz like a garage door openers, car remotes, etc... but Z-wave I believe is at 908MHz so has greater range but slower data.
So - the real question is - what works? Well, slow data means slower to react whereas faster data means faster to react, but again, if you are at too great a distance, no data is transmitted, so slower data stream with longer range may be better (but faster would be better if possible).
Both Z-wave and Zigbee can work over a greater range provided they can "network" to enough of their own devices and provided all those devices will network with each other. You can surmise then that Z-wave is going to be your best shot for longer distance links where you have fewer networked devices (relaying devices) nearby. Since power consumption and device transmission power go hand in hand, weaker batteries, weaker/shorter distance so keep that in mind also with devices in longer service or higher usage. Encryption further helps but hinders this problem as well because now more power is required for decryption (but necessary to protect devices in the network).
Unfortunately, because so many other devices operate in the same frequency ranges including phones, tablets, pc's, sensors, etc. and a neighbor's house may also be close enough to "network", you could end up with interference or confusion. I am reminded of a wireless hallway contact sensor that thought it was being jammed whenever the neighbor's motion detector sensed motion. A real pain to troubleshoot btw.... Your Z-wave devices need to "see" only the ones you want seen.
I like the diagram on the Zigbee site that also provides some specs. Take a look at the pic.
http://zigbee.org/zigbee-for-developers/network-specifications/zigbeepro/
Hopefully this will help you resolve the issue you are having but sounds like you have a mess.
