I have both Ubuntu and Mint on a USB stick to run them on any PC. It works great.
You only need about 4 GB USB stick for each. That is also another way to run them without installing anything on your PC and you can still use it and save files and whatever you might need. Of course with a 4gb stick, it limits you to about 2.5GB of free space but it's good enough for me to test out.
This is a very good point that I don't think has be mentioned yet. Whichever distro you choose to try will have a Li ve version (usually). Just load the ISO on a flash drive using Unetbootin or a similar software (USB boot drive creator) and try it out without commitment.