It seems that many of us, as noted, end up on Zigbee channel 20 or 25, with some variations, and try to keep our 2.4GHz Wi-Fi down at the other end of the channels, on 1 or 6. Separation of Zigbee and Wi-Fi is good. 
Really depends on your overall 2.4GHz Wi-Fi environment as well as your devices. If you have nearby neighbors blasting their 2.4GHz Wi-Fi at full power/40MHz on Wi-Fi channel 11 and you can't get them to adjust anything, you may be happiest w/Zigbee on 15.
