First, this is in addition to some of the other considerations/features already mentioned. Second, this is not a sales pitch -- I work only with F1000 companies. Third, I am somewhat biased in that we are a partner of Shavlik. However, we are also a partner of Microsoft and have designed solutions to complement SMS etc. We chose Shavlik because they were simply the best "Microsoft" patching solution.
Now, that brings me to the all important piece I have heard no one mention yet (I may have missed it -- sorry to anyone if I did). Most likely, you are going to want a patching solution that will address patching for some of the other ubiquitous programs like Adobe right? With WSUS you would be SOL.
Shavlik new release covers things like Adobe and Firefox (HFNetChkPro 5.1 --- GA in about 3 weeks).
Here is a clear example of the limitations of WSUS being an MS-only solution: Here is a pull from a forum on the latest Adobe vulnerabilities.
Adobe Systems on Wednesday rolled out patches for security vulnerabilities found in Adobe Reader 7.0 and 7.0.1, and in Adobe Acrobat 7.0 and 7.0.1.
The hole in the products, referred to as an XML External Entity vulnerability, under certain circumstances allows XML scripts to be used to discover a user's local files.
According to Adobe officials, the vulnerability is within the Adobe Reader control. If an XML script is embedded in JavaScript, it is possible to discover the existence of local files, according to a security advisory from the company. An attacker could then maliciously use the gathered information. But the statement pointed out that the local files can be found only if the attacker knows the complete file names and paths in advance of such an attack.
The vulnerability impacts Acrobat and Reader running on Windows and Mac platforms.
Adobe recommends that Reader and Acrobat for Windows customers download the updates provided on the Adobe Web site here.
To clarify - PA+ *will* cover things like Adobe Reader and Firefox, as soon as Shavlik the next release HFNetChkPro 5.1 in 3 weeks My understanding is that this product also addresses/runs on Linux according to their website
The product is also very easy and much less money than most of the other Enterprise Patching Solutions. Yet, It is the TCO that really makes the difference. While WSUS may seem like the least expensive product, consider the amount of time you will still be spending addressing patching & security issues from Adobe and all the other non-MS products that your 300 or so desktops will need. Hope this helps