The decision about choosing between both bands should be based on the fact that 2.4 GHz is susceptible to wireless interference (cordless phones, microwaves, Wi-Fi networks) and 5 GHz is mostly affected by walls and solid objects.
If you would like to set your router to a fixed channel in 2.4 GHz I suggest you using the least crowded channel by checking your wireless environment with an application such as WifiInfoView, NetSurveyor, or inSSIDer (version 3 was free).
You can get more information about this subject here: http://en.wikipedia.org/wiki/List_of_WLAN_channels