This topic is old, see bottom for an update 2 years on.
Purchasing wireless equipment can be a bit tricky, as there is a chance any off-the-shelf adaptor (especially the latest models) won't work out of the box (due to no driver being in the kernel/repositories) or even worse... it has a chipset that isn't even supported at all!
So if you've got a 5 GHz / 2.4 GHz router and in need of something that can reliably transmit through 2 floors in a house while sustaining decent speeds to the network - this 2016 adaptor is a good candidate:
What did I find?
Asus USB-AC68 Dual-Band AC1900 USB Wi-Fi Adapter
- USB 3.0
- 1300Mbps - 3-stream 802.11ac device (433Mbps per stream),
- 600Mbps - 3-stream 2.4GHz 802.11n (256QAM) = 1900 Mbps class
- Dual antennas -- (my dad describing it as "alien" looking
)
Bought from eBuyer for £57.99.
Technically known as:
Bus 001 Device 006: ID 0b05:1817 ASUSTek Computer, Inc.
(Two floors below, my router is a TalkTalk Huawei HG635 VDSL 802.11ac)
This adapter won't work out of the box, but that's OK!
With a bit of research, this ASUS USB-AC68 has the Realtek RTL8814AU chipset.
Wikidevi is very helpful in identifying whether a driver is in the upstream kernel (e.g. works out of the box), or if there is or isn't a known driver that will allow the device to work.
Luckily, I found a driver in this repository and had a good feeling I'll be able to compile it once I order it. (It delivered, it worked! 
)
I did come across a few other repositories that provided a driver for the same chipset, but this one seemed more up-to-date and promising.
DKMS Support
This driver has DKMS support which will save headaches later. This is so the driver can be re-compiled when the kernel is updated, otherwise the device would stop working when you update your system.
What commands did I use?
sudo apt-get install git dkms
git clone https://github.com/nazar-pc/RTL8814AU.git
cd RTL8814AU
sudo ./dkms-install.sh
sudo modprobe rtl8814au
Is it future-proof?
This particular driver repository was also kind to specify which versions it worked on:
Should work on modern Linux kernels from 4.9 to 4.14-rc2 (older and newer kernels might work as well, but were not tested, feel free to let me know if it works on others).
Currently being on Ubuntu MATE 16.04 LTS with the kernel at 4.10.0-30, I have faith it'll continue to work from this point forward. 
Edit: 2 years later...
Things are starting to get frustratingly buggy. If I disconnect and reconnect to the network, I'd have a considerable amount of loss packets - which causes pages to stop loading. A ping test to the router confirms this.
Bus 005 Device 003: ID 0b05:1817 ASUSTek Computer, Inc. USB-AC68 802.11a/b/g/n/ac (4x4) Wireless Adapter [Realtek RTL8814AU]
I have heard that Realtek are not a reliable brand, and even one of the repository forks would advise against buying hardware with this chipset. I agree.
I'd highly recommend looking for hardware that have support in the kernel, even if that means deeper searching to find a dual-band adapter. It's been solid when it works, but for now, I'll be switching to Powerline Ethernet adapters.
Till then, we’ve got to rely on community efforts to get hardware working – especially more modern hardware like dual-band Wi-Fi adapters.