USB keys are known to be too unreliable in the long run, so my advice is a big STAY AWAY from them; but I can testify an external box, connected via a USB2 / USB3 cord, plus an (Intel 530 series) SSD can do wonders as a portable, full GNU/Linux system (Kubuntu in my case). Boots to desktop in under 1 minute even over USB2 (USB3 is a lot faster), and is very usable despite the USB bottleneck (
way better than any USB stick). 120Gb, even after installing a full distro, leaves you with plenty of space to carry along your favorite music, virtual hard drive images, so you can carry along your Windows virtual machine(s) intact, and already configured [1]. You can then use TimeShift (or manual rsync cronjobs) to backup your full system to local drives/USB keys, and sudo apt-get upgrade the system whenever you get a reliable internet connection.
Minor issues I have experienced with this setup:
1. The biggest issue of all is, due to the peculiarity of USB power design, most SSD's might experience failures when the machine is powered off. This is a known issue with USB keys too! Believe me, booting a system every time not knowing if it's going to end in the dreaded fsck mess with thousands of errors really is a nightmare. Going with the Intel 530 series SSD's (which is a known solution) fixed the issue as they have larger capacitors, though Samsung SSD's might be reliable too.
2. USB power: on most laptops, your single USB power cord is unable to power the external drive; you might have to use the popular Y (double connector) USB cable.
3. Stay with open source video drivers. Especially swapping nVidia <-> AMD drivers is a recipe for disaster.
4. Audio HW devices will stack up, possibly resetting their settings when you return to a previous machine.
5. You might sometimes have issues with UEFI when you first boot your system on a new machine.
[6] VirtualBox VM's have sometimes issues when the system unexpectedly swaps eth0<->eth1 interface names, but this is easily fixed in the network configuration before booting them; also swapping AMD and Intel processors is risky, and can unexpectedly hang your Windows system on boot; BTW don't forget to carry along on your external drive every hard disk image they might use, or the VM's won't boot. Of course this applies to Vagrant boxes too, but is less relevant as you can destroy/recreate the box any time.