Should change your poll slightly... or at least offer multiple selections...
I prefer FreeBSD machines for low-end, x-86-based servers - up to 4 CPU boxen. They are rock solid, and do their job without the need to be fancy. They Just Work.
However, I prefer Linux for workstations, anything that I'm going to need the latest in drivers for devices, or graphics, et. al. It's good on he desktop, and getting to be a better-than-halfway decent alternative to Windows on the desktop for all of your main business functions.
And, at the same time, I prefer Sun boxes for anything that requires high-uptime, excellent onsite support, and anything enterprise-class that requires huge processing power or RAM, anything over 4 CPUs, or anything that needs to run an Oracle database.
I don't feel FreeBSD or Linux is right for large, enterprise class computing (yet), since it's limited by the hardware it runs on. Oh, and don't give me any argument that a 2 or 3Ghz Xeon with 2MB of cache is better than an UltraSparc III CPU running a 1.1Ghz, with 8MB of cache. It just don't measure up. Sure, webhosting, fine. Data warehousing, dream on.
FreeBSD is making progress with it's port to Sparc architecture. I would certainly run FreeBSD on Sun boxes for large applications. The worst thing RedHat could have done was stop offering a Sparc port with it's final 6.2 release... it's limited itself now to x86 hardware. Maybe with Opteron or IA64 it will get better...
Sure, you could use a cluster of x86 boxes, but how much complexity does that add, and does it really make it worthwhile for your solution? You end up spending a comparable amount for a cluster of Linux boxes, when you could just buy a 12-way midrange Sun box and get the same or better result. Clustering is generally a great solution when you're aiming at the supercomputing functions, where you need a helluva lotta CPU and not much else... like in scientific or mathematical research, not when aiming at a midrange Oracle data mining project.
And, on the same token, Windows is good for playing games. That's about it. Don't even think of suggesting Windows as a 'server' platform for me. It is not now, and will never be enterprise class, until it is rewritten from the ground up, and not written for button-clicking morons. I use it on the desktop, but that's where it stops. Even my file and print serving at home is run off of Unix boxes with Samba.
Everything has it's uses to some degree... it's not like I prefer OS #1 to OS #2 and only OS #1 for everything, it's the best and there's nothing else better. For a single nail, do you use a hammer, or a sledge? When sheetrocking a room, do you use a screwgun, or a manual screwdriver? Each one's got it's fit... and each Unix is better in the datacenter than any Windows box could ever be.