[Oni]

Of the two, definitely Kali, because it's more streamlined to use.

I'm sure you've heard me say this, but I'll say it again. Downloading a hacking distro is stupid, because you will probably never use over half of the tools. Install what you need on your favourite distro, be that Ubuntu, Debian, Fedora, Mint, or whatever else. You'll learn how to properly use a small set of tools, which is better than knowing a little bit about a lot of tools.

If you need it as a liveCD, you can always convert your tooled-up operating system into an ISO, probably easiest done by editing the SFS in the default ISO.

Personally, I'd use it as a live-disk. That's what I've normally done. I'd never use it as a main operating system.

Kali is essentially backtrack 6. The only major difference is that Kali is using Debian 7 with GNOME 3 instead of Ubuntu with GNOME 2.

You can easily change the desktop environment, but other than that, yeah.