IOS/MacOS X - the native clients track more than just RSSI - they also track CINR, as such, they make reasonable decisions on which band/AP to select - and the clients do track other AP's/Radio's within the same SSID set - this is not dependent on having Airport Extreme's or Express's as the AP - this is general guidance overall.
Breaking out the AP's/Radio's into different SSID's will effect the user experience in a negative manner there.
In my experience, Win8 and Win7 behave similar when using the native WiFi clients - third party SW may work, might not - Cisco Clients are pretty good about it, as is Intel's client SW - ASUS's SW for the USB-56AC isn't very smart, as it tracks the MAC addr from my experience, rather than the SSID - so it's a bit confused when offered mutliple AP's on the same SSID/band class..
Android 4.0 and later - the native client provided by Google seems to be fairly smart - third parties can get into trouble if they replace the native client with their own - this is for both OHA compliant (e.g. Google Services, etc) and AOSP (Android OpenSource Project, e.g. not OHA compliant).
At the house - I run an Apple Airport network with 2 AP's - common SSID across both bands - I mostly work from home these days, and I do a lot of VOIP calls with a SIP client on my iPhone (
CounterPath Bria, very good client) or MS Linc 2013 - walking around the house as I typically do, the handset will transparently handoff from AP to AP and is band class aware - just works... and this goes for both Android (Samsung GS4) and IOS (iPhone 5 and iPad Mini Retina).
At the office - current job runs Aruba - again, common SSID's (we run two, CampusWIFI, and CampusGUEST) and no problems here with Windows PC's on Win7/IntelWiFi - Macs, and a mix of Android/IOS handsets and tablets.
Previous job a few years back - Cicso Aironet's A/B/G, again, common SSID's across both bands.
Again, everyone's situation is different - and this is why most vendors (including Apple) offer the means to use different SSID's for the 5Ghz radios.
My recommendation is to keep things the same - and focus on where the clients will be at, locating the AP's for best coverage across both bands.
sfx