It's safe to leave the bandwidth setting to default, unless your surrounding have too many wifi access point. which 40mhz might make it worst because it's wider.
by set it to 20mhz only, some of your wifi devices speed will be downgraded to 72mbps or 150mbps, instead of doubled bandwidth.

N = 802.11N, well unless someone connected to your wifi router still using older wifi device which only support A,B or G; Force N mode is fine too.
If set to Auto mode means your router will work a bit harder to support older wifi standard, too.