Hello,
The 2.4 GHz spectrum has only 3 available non-overlapping channels (1, 6, 11), so if you are in an area with other wireless networks in range you could see some interference. There is also potential interference from Microwaves, Bluetooth, etc.
The 5.0 GHz spectrum has many more channels available, and the chances of interference issues are much much lower.
The range is typically better on 2.4 GHz and it is more compatible with devices, but 5.0 GHz will usually have better performance. Also, with both radios turned on, you can split your clients onto both radios, so they aren't contending with eachother as much, since wireless is after all a shared medium.
No real downside to having both Radios turned on that I can think of, aside from a minimal power draw increase (not sure on the specifics)...
-Pat