nUbuntu was born as a security testing xUbuntu flavor. However, due to the too big effort needed to keep a whole distribution up to date, despite it being based on the work by Ubuntu teams, it is seldom updated.

Much more frequently are updated its core components, through the nUbuntu PPA. Adding this PPA to your packages sources /etc/apt/sources.lst (or through the graphic tool in System -> Administration -> Software Sources) you can get the best open source security and penetration testing tools on your own *Ubuntu flavored desktop with much less effort than having two separate OS.

Here is the PPA page where you can find the PPA strings for your own *Ubuntu version.

https://edge.launchpad.net/~nubuntu-repo/+archive/ppa

deb http://ppa.launchpad.net/nubuntu-repo/ppa/ubuntu YOUR_UBUNTU_VERSION_HERE main
deb-src http://ppa.launchpad.net/nubuntu-repo/ppa/ubuntu YOUR_UBUNTU_VERSION_HERE main

Signing key: 1024R/AA7D4E90

[in order to automatically add the signing key have a look at this post]