Although you speak of installing, the Persistant Live mode is easier to use and setup. I do not know about really installing to USB - this should theoretically be possible, but I doubt it will be much faster than the Persistant Live stick, which is painfully slow.
Ubuntu however does not take up much space; I personally use Xubuntu AMD64 and Mint Xfce x86, both with KDE runtimes and LibreOffice installed, and neither has grown beyond 6/7GB.
So I'd recommend you to just use it from the SSD.
But, since you'll need a Live USB stick anyway, I'd recommend you to just use this tool and take any version of Ubuntu you want to try for a test drive: http://www.linuxliveusb.com/en/home ...and take it from there. I've been using it for ages and have achieved better results with it than with unetbin.
Do note that to use the USB stick in persistant mode, you will need to add persistance in the LiLi tool. There's a big slider there to set the size of it. Just use whatever you have available.