If your question is specifically on vuln scanners, then Nessus is just fine. Doesn't have the bells and whistles as some of the big boys like ISS, LANGuard, etc. But considering that many companies have nothing, making yourself knowledgeable in Nessus will do you well. Plus, if being hired is your concern, I don't think anyone will look down on you for only knowing Nessus. Not knowing any vuln scanner can hurt you.
As for Core Impact, it is an automated pen testing tool and much less of a vuln scanner. According to their web site:
CORE IMPACT is the first automated, comprehensive penetration testing product for assessing specific information security threats to an organization. By safely exploiting vulnerabilities in your network infrastructure, the product identifies real, tangible risks to information assets while testing the effectiveness of your existing security investments.
That being said, I've played with it, and it is awesome. If you do pen testing for a living, it is worth the money. If you are just learning or have a very tight budget, look to Metasploit.
Hope this helps,
Don
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OSCP - Offensive Security Certified Professional : Offensive Security Releases Sample Pen Testing Report
