How to Choose 20 or 40 Mhz?
If you use 2.4 Ghz broadcasting radio, you should use 20 Mhz for the channel width. The simple reason is that 20 Mhz is really a supportive measure for your older devices. Using 20 Mhz on a 5 Ghz radio setting defeats the purpose of actually using 5 Ghz radio setting.
If you use 5 Ghz broadcasting radio, the chances are that your network is only consisted of the latest devices that support 802.11n. This is when you should use the 40 Mhz bandwidth.
Only use 20 Mhz / 40 Mhz combination if one of your devices require it. If not, just set it to 40 Mhz.
How to Control Channel
The best channel band to use are 1, 6, 11
The best channel band to use are 3, 11
You should use 40 Mhz instead of 20 Mhz, or use combination if your device needs it and the router supports it.
Any channel with the least amount of interference will do. The chances are you are free to use any channels. Consider using the auto function so that when your router can auto adjust for the best channel to use.