Which means that, as an end-user, you have no way of knowing for sure if your device will be making any intelligent decision, since every manufacturer can decide on which criteria to base such a decision on. And that's in addition of those hidden settings that I mentioned where you can tell it which band to "prefer", assuming you knew where to look to find those settings.
All the more reasons to take it into your own hands by using different SSIDs - only way to be sure that the correct decision will be taken.
Seeing how poor of a job router drivers are doing at selecting the ideal channel when leaving things on "Auto" doesn't inspire me any confidence in their ability to make the best decision with the client either.
Except in my experience letting the client decide (in the absence of band/WAP steering from the base stations) instead of forcing a client to be on one set band is the best way to do it.
5GHz, with the exception of open air, has less range than 2.4GHz, but almost always has better performance than 2.4GHz at short range, even in the case of uncluttered EMI environments, especially when you are looking at 20/40MHz on 2.4GHz 11n versus 40/80MHz with 11ac. However, a lot of times AP density is not such that there'll be good hand-off between the APs on the 5GHz band before 5GHz performance drops below that of 2.4GHz performance.
Granted, I have one 11n and one 11ac base station inside my house, but with that, 2.4GHz is faster in some parts of my house than 5GHz is. If I set a unique SSID for 5GHz, my devices will NOT roam over to the 2.4GHz band when in those areas and stay with 5GHz, even though it is slower than 2.4GHz. However, with a unified SSID I DO see them roam over to 2.4GHz most of the time that 2.4GHz would be faster than 5GHz. It doesn't always happen, but it DOES happen some/most of the time. In the converse, I will say that IF one of my clients roams over to 2.4GHz, it rarely moves back to 5GHz when close to base station, I generally have to roam to the other base station before it'll decide to switch back to 5GHz.
Exceptions, if I set the WAP to 20MHz 2.4GHz, then devices are much more likely to roam back to 5GHz after a minute or so of being close to the WAP (I have it set to 40MHz 2.4GHz and 40MHz 5GHz). Our iPhones and my wife's iPad 2 ALWAYS roam back to 5GHz given 2-5 minutes of being close to the WAP (this may be in part because I know my iPhone 5, and maybe her iPhone 4s are both 40MHz capable in 5GHz, but only 20MHz capable in 2.4GHz...no idea why the iPad 2 does it as it is only 20MHz capable in both bands).
Of course for max performance, I keep the WAP on 40MHz 2.4 and 5GHz, because the WAP covers most of the house where I am more likely to want performance for my wireless devices. The router, though faster...is a better actual router, so it stays in the location even though it is >>>faster on 5GHz (and actually slightly faster on 2.4GHz too if I set it to 40MHz mode) than my WAP is. So it lives with 20MHz 2.4GHz and 80MHz 5GHz, so as to not overlap the channels of my WAP.
Devices there almost always roam to 5GHz once they get close to the router even if they were on 2.4GHz before. Mostly, but not always though.
In my front yard for both the WAP and router, devices almost exclusively connect to 2.4GHz since the 5GHz signal is so weak in comparison once you get outside of my house. I have a 2.4GHz only WAP covering my backyard, though I plan to replace it with my spare WDR3600 at some point (I want to see how the WAP fares over the winter, even though I plan to leave it unplugged through the winter as I won't really need wireless access outside from November-March, and my indoor WAP does cover my deckwith 2.4GHz, just not as great as I'd really prefer (around -68 to -76dBm depending on where I am standing on the deck)).
Which that also goes back to the seperate SSIDs...if I had devices "stuck" on only the 5GHz, they'd have (DO have as I've tested) a heck of a time when stepping outside. Pretty much the only time they'll switch over to the 2.4GHz SSID is if the 5GHz SSID disappears, which often results in VERY non-seamless roaming as I drop the wireless connection completely a lot of times for several seconds before it'll connect to the 2.4GHz SSID, for example going outside and stepping more than a few feet from my house (that -68 to -78dBm for 2.4GHz on my back deck with the indoor WAP is -86 to zero for the 5GHz signal, front of the house is slightly better, but still around -83 to zero once I get more than about 15-20ft from the front of my house it disappears completely).
My clients are iPad 2, iPhone 4s, iPhone 5, Asus T100, HP Envy 4t on Windows 8.1 with an Intel 7260ac in it, Asus Memo Pad HD7 (two of them, running 4.2.2 I think), which doesn't include a few other wired/wireless devices that exist as wired only (other than some testing, IE Apple TV, network printer and one or two others). In all, they all generally work extremely well with a unified SSID across bands and across WAPs/router.
