Apparently OpenVAS originated as a fork of Nessus. It is very easy to install and use OpenVAS because it's, well, open. However, am I kidding myself if I just use that instead of Nessus? Should I be using both, or if I use Nessus then is OpenVAS surplus to requirements?

To break it down into non-subjective sub-questions: * Is openvas a superset or subset of nessus? * Is one updated more often than the other? * Does one have a bigger vulnerability database than the other? * ...or are there other qualitative differences that I may be missing?