I'm not going to get too into this...but here are at least some obvious advantages.
My personal environment right now is a small Mandrake10/Redhat9 cluster...
I am running a virtual hosting environment on both, with load balancing for web traffic configured.
These boxes also run DNS/Mail servers/MySql Servers/IDS's, etc.
Other than my personal time, the only cost has been hardware, and that has been nominal as linux will run well without the GUI on a toaster. The uptime has been unparalelled thus far, I think I posted somewhere else in the forums that one of my servers has not been rebooted since the last clean install(about 6 months ago), that was when I migrated one box to Mandrake 10. I have not found a software yet that is linux based yet that will "make" you reboot to complete the installation. This is all too common in windows environments and leads to downtime. There is also the obvious cost effectiveness of stable free software. The only thing we pay for is verisign for credit card transactions and our bandwidth. The OS, daemons and tools are completely free, and relatively easy to maintain to the experienced user. As far as clustering, there are many free cluster software alternatives available depending on your industry. Many made specifically for medical research and other disciplines. There are also many free cluster distibutions available. For simple clustering you could even look into something like clusterKnoppix, which is a "live" CD OS for an instant cluster...you don't even "need" a harddrive. Other projects that I have run across are OSCAR(Open Source Cluster Application Resouce), which you can check out here -->
http://oscar.openclustergroup.org/tiki-index.php and you may also want to check out the Beowulf community--->
http://www.beowulf.org/ and ROCKS --->
http://rocks.npaci.edu/Rocks/ There are many more out there.
On the security end, most of the hits that I pick up with my intrusion detection system(ACID/Snort) are IIS hacks, which, if you are running linux you are'nt really vulnerable to. Also on the virus end of things linux is much less likely to be compromised.
Now don't get me wrong, there is quite a learning curve to linux, and I'd think that you would want to take that into consideration as well. If you are completely "green" and have never used the OS before I would think it would be nightmarish to jump straight into a beowulf cluster. But as far as deploying simple web servers with Mail/MySql/DNS it's really not all that hard. I don't want to make this too long but hopefully this gives you at least one honest testimonial to look at.
My background = I've been using linux for about 6 years now and I'm 31 years of age. I do all of my own IT work and am currently seeking employment in the industry. Learned everything I know from googling it and frequenting message boards. Just saying this so you know the resources ARE there even if there's not much commercial support.
If you have more questions about specifics or anything else keep posting and I will check back, not to mention that there are many very advanced users who frequent the site and can probably give you even more info than I can.
Alrighty, have a good day and best of luck to you.
linux_terror