2013 MBA with 802.11ac won't choose 5GHz even when faster

Question

I have a 2013 MBA and new (802.11ac) Airport Extreme.  With the base station in dual mode (2.4 and 5.2 networks sharing same SSID), the MBA refuses to connect to the 5GhZ network most of the time, even when very close to basestation.  This is annoying for wireless Time Machine backups and other high-throughput use cases (files copies to/from network are another).  Without moving computer, when I set basestation to use diff name for 5GHz network and then manually select it, transfer speeds go from ~12MB/sec to >40MB/sec (impressive!). 
Using iStumbler, I see that the 2.4GHz network indeed shows "better" signal levels, BUT IN THE REAL WORLD the 5GHz link is MUCH faster.  What's up with that?
I'd like to go back to dual-mode just for elegance and simplicity.   Plus, it seems like a bug.
My question is:  Can I force the MBA to always link to 5GHz network, or at least its propensity to pick it despite 2.4GHz link (falsely) appearing to be better?
Thanks!

Answer 1

I actually install large wireless networks professionally, so maybe I can help clear up a few misconceptions.
To the original poster, there really isn't a way at this point to force the MBA to select the 5.0GHz network, but this really isn't a "bug" of the MBA.  It's really a missing feature from the AirPort.
Your assumptions on how the client device works are correct.  They select the band based on the signal strength (basically Signal to Noise Ratio [SNR]).  This is actually a function of the NIC itself and is based in the hardware.  Since the vast majority of client NIC cards are manufactured by a couple of companies, it will probably take them a while to change the operation, and for marketing reasons, I don't know if they want to at this point.
The vast majority of people that I encounter every day don't understand that there's more to wireless performance than signal strength.  They look at the corner of their device and say, "I have 4 bars, look at how great my network is.  Yeah, it's a little slow, but that's just how wireless is".  Companies that sell consumer wireless access points are able to say "Cover your house with a single device".  I really think that the lack of consumer awareness is what causes trouble for those of us (like the original poster) that understand that there's more to it than that.  Even OS-X falls into this marketing bubble.  Why do I have to hold down Alt and click on the wireless icon to actually see performance data?  In my opinion, the icon should be the Transmit Rate as a number and I hold Alt to see the RSSI when I need to troubleshoot, but that would scare most people.  For these marketing reasons, I don't think that you will find client devices selecting 5.0GHz by default until 802.11ac really gains in popularity.
The other option, which wouldn't require a tremendous amount of work on Apple's part is a feature that many enterprise wireless manufacturers refer to as "Band Steering".  Basically, the AP (AirPort Base Station) will ask the client for its capabilities.  Once it knows that the client is 5GHz capable, it pushes the client over to that frequency and there isn't anything that the client can do about it.  If I knew how, I would be requesting that feature as an advanced option of the AirPort product line.  That way, those of us that really care about wireless performance are able to get it without having to hack through and broadcast a 2.4GHz network that will basically be unusable in the coming years since the new wireless standard is 5.0GHz only.