PC is the 80% solution, Mac is the 100% solution.
PC are open, the competition makes them cheap, and they are cheap (as in, there is software and hardware out there that is sub-par). You can upgrade a PC cheap, you can find a way to hobble together your network with band-aids for a few bucks. You can find ten times the amount of software for a PC then a Mac, and some of it is actually half-good.
Mac's are closed, it either works great or doesn't work at all, you're NOT likely to find some cheap hardware or software to create a solution, you can't write or have written some cheap software yourself. BUT, since you don't have a bunch of sub-par stuff being band-aided into the system, it works really well and doesn't have as many problems. Of course sometimes the answer to an upgrade problem might be, well you'll just have to go out and buy all new Macs.
Yes, I'm over-simplfying, you can post 10 pages of exceptions of what I just said, but the point I'm trying to make is, PC being more open architecture has more flexibility and options then a Mac, and Mac taking the opposite tact, being more closed architecture has less flexibility and options. BUT, that flexibility and more options results in crappy stuff in the mix and people keeping outdated equipment still running and messing everything up. The closed archicture doesn't have to suffer that stuff, thus more reliable and better working. Of course, also more expensive.