This is a problem I've had in my case - I have a couple of machines that are just at the edge of the 5GHz network reach, and I can see from logs that every couple of minutes it would switch back and forth between the 2.4GHz and 5GHz networks when using a single-SSID setup. I'd like to say that it was not noticeable from the end user perspective, but it was...
The other side of the coin is that, on those same machines in a dual-SSID setup, I find myself frequently manually switching back and forth between them as the signal quality tends to vary quite a bit.
I'm in an urban environment - lots of apartments all around me. When my router scans for access points, it can see 57 stations on the 2.4GHz band, and 29 stations on the 5.0GHz band... and that's just on the first pass. On 2.4GHz, they're evenly split between channels 1, 6, and 11. On 5GHz, they're evenly split between 36, 149, and 161.
157 looks as though it would work best, as I believe it only switches between 1 above and below, but it might be 1 above and 2 below (I can't recall which).
Depending on the strength of those 9 on 161, a 5dB or 7dB might result in those neighbors experiencing reception issues with their own routers, but a 2dB antenna chosen specifically for your current environment would likely help.
WiFi antennas don't operate in the way most would picture, as WiFi antennas are designed to broadcast more vertically, more horizontally, or about evenly on both axis'
So, when I set it to 157 with a bandwidth of 40MHz, OpenWRT shows it as broadcasting on channel 161, where if I do 20MHz, 157 shows as 157. Does this have something to do with the channel being positioned relative to the bandwidth requested?
If my upstream cable internet connection is only 60 megabit, is there going to be any real difference between a 150 megabit and a 300 megabit wireless lan? I don't do any high bandwidth traffic between wireless clients, everything is going out to the internet.