Quick hit I hope here.

Our auditing department uses ISS and NESSUS to scan our network. The company supports us using ISS to scan but not NESSUS. So a few things me'ah:

1. Is NESSUS a better scanner overall than ISS or should be used as a 2nd opinion scanner?
2. What about SNORT? Is it comparable to ISS/NESSUS? Is SNORT betta?
3. Is there just a host scanner to have a server scan and report on itself (or just use ISS/NESSUS)?
4. I see that nmap is supposed to be a part of NESSUS as well, has anyone seen big issues with using nmap? Either alone or as part of NESSUS?

Ok, maybe not such a quick hit, but I figure we have some pretty big brains out there...

Thanks in advance!