Uhm...
This is apples to oranges, really.
If you're looking for a superb file server and service provider with extensive use in a business setting, go with Unix. However, you must have deep pockets for a few reasons.
1) Liscensing and support
2) Hardware
3) Greybeards
Linux can provide most of the versatility of Unix (linux is not the best web server), but it costs less. All you pay for here is the support plan (a good idea). You'll have more flexibility in your staffing as well as you don't have to have Greybeards in order to function properly.
freeBSD (or any of the BSD flavors) are very good at their specific tasks. ie., freeBSD for web serving. can't remember the prefix for it, but there is a BSD version that is EXTREMELY secure (hasn't been hacked in over 8 years, iirc). there's another one too...but I can't remember it off the top of my head.
another advantage to freeBSD and Linux is that they are open source and heavily user tested. mixed bag, if you ask me...but for a savvy businesss IT department it can shave thousands off a budget. Linux and BSD have the distinct advantage (in the cost department) of running on consumer level i386 architecture...so you're not selling your soul to the likes of Compaq and AT&T.
Windows is a newcomer to the business end of things...at least as a server platform. The advantage here is that your business servers are going to be on an interface that 95% of the population is familiar with. The disadvantage here is that your business servers are going to be on an interface that 95% of the population is familiar with. I wouldn't want anyone other than an IT staffer working with the server...for obvious reasons (just imagine a couple of people in sales gathered around the server after it 'broke'). other than that...haha...I'm not sure that I'd use Windows at the core of any business due to its extensive vulerability. Since it is the most commonly used OS at this point, it's worthwhile for programmers to work out the exploits and stage attacks. Unix, BSD, and Linux all have a learning curve and they are much, much, much more configurable.
I am by far no expert on all of this...but from someone who has at least been in the trench...there's my $0.02.