I've experimented a lot with this. The main advantage is portability, the main disadvantage is performance.

If you are really handyman the most extreme combination is to generate a virtual machine, say in vbox and when creating the hard disk for this machine point it to the usb, although you must first make a whole set of partitions on the same usb and add the virtual machine runner on the same USB.

I did it with an ubuntu 14.04lts and it takes a long time to load but if the host machine has enough memory, and what you are going to do is not mission critical, then you don't really notice the lack of performance.

The icing on the cake was to generate an alias for documents and desktop to back up manually to the cloud (Dropbox).

It's great fun to see your users open their eyes when you stick your USB in and 40 seconds later, be it Mac or Pc it stops being Mac and becomes your machine.

I recommend the partition, with a small one of 20 or 25 GB for root and a little for swap is enough.

If you do it virtualised you will not find it very fast and you may not like the experience. Unless you have a powerful computer with good memory and an SSD drive.

If you are only going to try several linux distros to see which one you like, use the USB or CD-RW, I recommend that once you decide which one you are going to use leave it installed on a partition, because its performance and performance will be much better, now that you have the distro in the partition (fixed), again create a bootable USB of that distro for when they are away from your office or personal equipment called computer or laptop, you can use your same system on any computer outside, and do not leave data and information on a public computer, personally that what I do, best regards