After seeing that there have been a number of searches for Airport Extreme comparisons between the 2.4GHz and 5.0 GHz modes, I decided to augment the original table with iStumbler readings at 5GHz also. This is only for the Airport Extreme. With the DIR-825, the 5.0 GHz band was useless in rooms 5, 6, 7 so we turned it off. Those who are interested can interpret the results for themselves (or feel free to post your thoughts below).
As a follow-up, we have been using the Airport Extreme for two weeks now and I have been very pleased. The USB drive share has worked very well and I haven’t experienced any of the other weird symptoms that we had with the DIR-825. The weirdest symptom was that most days my iMac would lose its connection with the Internet and I would have to reboot. I tried changing the DNS from my ISP to OpenDNS to Google DNS (along with a variety of other things at the promptings of folks on the Apple forums). So far, with the Airport Extreme pointing only at my ISP DNS, the problem has not resurfaced. I’m not prepared to conclude that the problem is gone but it has certainly been nice not to have to deal with it for the past few weeks.
Also, after switching the MBP back to 5.0 GHz, it has been very responsive. Seeing how much better it works than the DLink, I’m going to leave it on and see how things go. If I start getting complaints, I’ll switch back and post here.