My router has 5 and 2.4 support. Can I give the same SSID to both networks and my device will log in with the fastest network it supports, or should I name the SSID's Home24 and Home5 (for example)?
Thanks for answering such a basic question.
However, the decision which BSSID (which band) to pick is done client side, there is very little the AP can do to influence this decision (in the enterprise market you will see some non-standard compliant bandsteering efforts trying to force the clients to a preferred band, but that not quite perfect either - and currently not available to OpenWrt at all). This means that the results aren't always perfect and that clients can stick to one band longer than what would be ideal, enabling IEEE 802.11r and using the same SSIDs can help a little to help the clients making automatic decisions. In other cases it might make more sense to force a client to a particular band by hand (5 GHz signals tend to have lower signal strength, but can still be faster than 2.4 GHz in the same spot). There is a lot of potential for tweaking this (e.g. lowering the 2.4 GHz txpower, to make 5 GHz more attractive), but it really depends on the local circumstances and requires testing with the different clients involved.
Just to add a real world anecdote to what @slh has said, I use the same ssid for 2.4 and 5ghz and do what @slh suggested, lower the 2.4 band and raise the 5ghz band a bit and it works pretty well in my environment (small home with router in central location), the devices that I expect to choose the 5ghz band do so about 80-90% of the time. It never even occurred to me to enable 802.11r so I'll have to give that a try.
Enabling 802.11r is a good idea in theory and should never hurt (practice sometimes begs to differ though, as there are broken clients around which don't work with 802.11r enabled), but only very few clients do actually use it (personally I do enable it myself).