But to connect at A it has to be at 5Ghz. So how can the 360 connect at 2.4Ghz and A? A is only mode of the 5Ghz band. And the router can only broadcast at one band at a time, so how can your laptop be connected at 5Ghz and your 360 at 2.4? So I am not sure how what you say can be true. Could you explain?
As for performance, 5Ghz has less range then 2.4ghz. Rule of thumb is the shorter the wave length, the less the distant. One of the reason AM radio has so little range. Now where 5ghz shines is it is outside the interference of other 2.5ghz systems. This includes some cell phones, some cordless phones, and other wireless network. So if your getting more range on 5Ghz on A, then your 2.4Ghz is probably very congested. A has huge down falls though. Not only does it not have the support for alot of the interference avoiding system G has, but it maxes out at 54Mbps. And even when connected at that rate, when you have signal problems, I have seen actual through put dwindle to 1-2Mbps.
So I would still suggest G and N at 2.4Ghz and make sure Auto Scan is on for the channel. If you see that the 360 gets random disconnects though, try setting it to a manual channel and play around with it until you find one with the most signal strength. Also, you may want to try enabling "Extra wireless protection" Under "Advanced Wireless". Cuts out the interference you would from other B networks in your area.