This is something that has been bothering me for a long time. I use the same SSID for both my 2.4GHz and 5GHz networks, so that clients can roam from one network to another. However, in this setup, the client decides what network it think will work best. And unfortunately, most clients just suck at making this decision.
Usually my clients end up connecting to the 2.4GHz network, unless they are in the same room as the AP. There's a simple explanation for this: the 2.4GHz network usually has a stronger signal strength, simply because the lower frequency penetrates walls easier.