The 2.4GHz will give you more distance. The 5GHz will give you more speed. Depending on your area, selecting a different channel may be better and depending on your devices a different channel width may be better. Most do fine with the auto settings. Router location is big. small changes can make a difference such a moving it a few feet one way or the other id you are having connection issues. try adjusting the antennas, turning the router different direction and different heights. Be sure to use good cables as they affect the quality of the signal and speed. Some get better results with NAT boost off and others with it on. Leave QoS off unless it is needed. A good modem that is better than your ISP tier helps also. Read this for more info https://www.cnet.com/how-to/home-networking-explained-part-1-heres-the-url-for-you/

@ArcherC8 Thanks for your thoughtful response.  A few follow-ups ...

 

> 2.4GHz will give you more distance. The 5GHz will give you more speed

 

Do you have an example of a distance at which there is a noticeable performance difference between 5GHz and 2.4GHz?  (E.g. after 50 feet it starts to break down)

 

> selecting a different channel may be better

 

Do you have an example of a situation where it may be better?  I'm not clear what criteria to evaluate this against.

 

> and depending on your devices a different channel width may be better

 

My devices are a MacBook Pro, an HP laptop, two iPhone 6s', and a NEST device.  If this doesn't help, what channel widths are better for which devcies in general?

 

> Some get better results with NAT boost off and others with it on

 

Hate to sound like a broken record, but do you have any examples?  That might better help me figure out what I need to do.

In my house with an Archer C8 V1 using Comcast (Xfinity) Blast 200 service, I can get over 200 on WIFI 5GHz in line of sight 10 feet or so, On the 2.4 I never get that high. Just ran a test with my iMac five feet from router 2.4GHz got 92.0 and 5Ghz got 235.3. Speed will go down with distance and objects that block it. Also you get signal interference with non WIFI signals from other devices such as wireless cameras, weather stations, cordless phones, ,,, On your Macbook, hold the option key down and click on WIFI icon in the upper right hand menu bar. Then click on the Open Wire Diagnostics. Do NOT click on Continue. Instead click on Window in the upper left hand corner, then on Scan. A window will open, Expand the window and you can see all the WIFI in your area and what channels they are on. If several are on your channel, you may want to try a different channel. Some devices like wide channels and others more narrow channels. Also interference from other non WIFI devices may affect it. A Wide channel may bleed to another channel. The reason there are many options on the router as everyone's setup is different. for example, you have a half a dozen devices and I just checked and have 23 wireless and 3 wired running with a few wireless devices turned off. If you are in an apartment complex, your needs will be different than someone with out a lot of neighbors. Run your own speed tests.

Wanted to report back.  Turned out there was no degradation at about 40 feet from the router between the 5GHz band and the 2.4GHz one.  In fact, while speedtest.net reported a download speed of 31 Mbps for the lower band, the 5GHz band was clocking in at 141 Mbps!