Retina vs. Nessus:

I've used both extensively on purpose: you cannot rely on 1 vulnerability scanner (as others point out).

Retina:
* Detects Windows vulns much better
* Will miss things sometimes (as does Nessus)
* No CSV or TXT export for reports

Nessus:
* Seems to detect web server vulns a little better
* A little tricky to configure server (UNIX gurus will do fine)

By the by, Retina's port scanner is Nmap which is darn accurate.

Use both but if I could only have one I would pick Retina (assuming I had the budget).

PS. Any Retina users out there who want to share tips/tricks/issues?