Which version is preferable for my computer?
14.04 LTS because it's more stable, and the 64 bit version because your computer is 64 bit, which will help with performance. Linux is not like Windows, you don't need a lot of RAM installed to run 64 bit. Plus 14.04 will be supported until April 2019. 15.10 will be unsupported in less than a year.
15.10 is newer & you may have problems that others can't help with. I've been running Linux Mint for 6.5 years, and still prefer the LTS releases (actually that's what Mint has gone to, all LTS ones).
For those who are new to Linux, I always recommend LTS distros, no matter which one is chosen to run. Kubuntu is fine if that's what you want. But some things you will need to relearn, KDE is different from MATE. That doesn't mean that KDE is harder, it just means you'll have to perform some tasks different & adapt to the new environment.
Also, running from a USB drive will be slower than on your computer, so if you're wondering 'why is it so slow', you'll know why.