LANguard is really more for vulnerability management.  It is also much more expensive than Nessus since it is a per IP license.  It saves the IPs you scan and once you hit the limit, that is all.  You cannot scan any new ones.  The other negative on it, is that it only supports a max of 3000 IPs.  So it is only meant for Small/Medium businesses as opposed to large enterprises.  Not that it really matters for testing and such.  I think you are required to configure it with credentials though, so unless you have creds for the external system, then it might not be of any use.  One major advantage of using LANGuard compared to Nessus is that it will deploy patches for you.  This includes Java and Adobe patches as well.  I've even seen it offer patches for Wireshark.  It is certainly a very versatile patch management solution and doesn't really require any major hardware to run.