http://www.speedguide.net/faq/is-5ghz-wireless-better-than-24ghz-340
So, essentially 5Ghz can theoretically be faster and has more channels so is less likely to suffer from interference, but it has a shorter range: at the same power, the distance you get is smaller at higher frequencies, and also solid objects are bigger obstacles.
2.4GHz is more likely to suffer from interference (obviously this depends on how many 2.4GHz wifi/BT devices you and your neighbors are already using simultaneously), but it has a better range.
In my experience, range is the important thing. Speed will not matter when you're using wifi for internet; if you're using it for file transfers on your home network (e.g. streaming stuff from a NAS) then speed does matter, but you'll never beat the speed of something wired anyway.
Of course, if we pay extra for the 5GHz then we still have the 2.4GHz bands, so I guess you could get the best of both worlds.
In practice, I would probably be using the 2.4GHz bands most of the time though, because for me, range is nearly always a bigger problem than interference or speed. Not much point in having a fast and non-congested 5GHz band when you can't see the access point...
Should we pay 14€ extra for 5GHz? In my opinion, it's not really worth it. I'd rather get a bigger/faster eMMC or something for those 14€. You can still be future-proof by upgrading your wifi router to a dual-band one and making sure that all or most of your
other devices (laptop, phone, smartTV, etc) use the 5GHz bands, which means the 2.4GHz bands get less congested and will work better again.
Though perhaps we could have the cheaper wifi chip in the devices without 3/4G, and the more expensive one in the devices that will be more expensive anyway? You could also offer all 4 combinations, but that is just going to complicate things. Most likely, people who are willing to pay for 3/4G will not mind paying also for the extra 5GHz band, while those with a tight budget will drop the 3/4G and they will certainly appreciate not paying for the extra 5GHz band. So if you're going to have two different main boards anyway, you can just as well make them differ in terms of the wifi chip too, right?