It depends on a lot of things. Two jumps to mind:
1. Currently, there's less things operating on the 5GHz spectrum, so it's a lot less clutter. But the reach is not as great on the 5GHz spectrum as it is with the 2.4GHz.
2. If you have multiple computer sharing a NAS, for example. Then the dual band would be ideal...so not everyone would be sharing the bandwidth of the single band. You can have half the users on one band and the other half on the 2nd band.