There was a time where it would be irrelevant; as the benefit would never be tangiby-felt, but now its starting to change with Videotron offering > 50mbit internet, video streaming, and high-bandwidth applications.
- If you're in an urban apartment building: 5Ghz only. This is because the sheer amount of 2.4ghz devices in your physical surrounding, that makes it literally impossible to have a fully-clear channel for your wifi to operate, and if you crave bandwidth; you will be best-served with 5Ghz.
- If you're in a residential neighborhood (without anybody stacked on top and below you) then, you'll want to use both 2.4Ghz and 5Ghz by having both networks using the same SSID (wifi name), and you might even want to consider a central location where to put your router (main floor, centrally-located, and forget the old basement computer desk that nobody uses anymore).
5Ghz allows you to reach must higher bandwidths (up to 1300mbps in some cases), while 2.4Ghz maxes-out at 300/450mbit, both in ideal conditions (which never happens; usually perfect condition flank speed is about half of that).
Also, there's a question of spectrum; 2.4ghz goes easier through walls than 5ghz. 5Ghz covers a single-bedroom apartment well, while it won't ensure an entire 2-story house, so thats where 2.4ghz comes-in.
If one is better than the other most or all of the time, you can go in your wifi connection settings and forget the other slower network, that way it will only ''know'' one of the two and won't switch between the two.
Usually; most wifi chipsets will select their band on the RX (receive) signal strenght (the loudest burrys the rest), however, more modern devices are leveraging extensions like 802.11h to make better band and power choices.
So I guess the simple answer is get a better router :-)
You should look at Videotron's next-generation wifi router; which has some interesting under-the-hood technology.