Macs have 50 percent less downtime and require 1/20th (or less!) the number of technicians supporting them at large installations.
Are you quoting the Apple commissioned IDC study from 1997 or is there anything more recent?
2. The Mac has superior colour calibration and typography controls/support than Windows. Still.
As I understand it Windows 7 colour calibration is equal to OSX. I know lots of professional photographers who work with Photoshop in a Windows environment.
I don't know about typography - any professionals I personally know have never commented on this being an issue with Windows.
3. Simply put, OS X. This is a huge advantage for creative workers over Windows in so many ways large and small I can't even go into it here. Again, there's a REASON why the creative fields are heavily Mac-oriented, and it isn't because they like shiny things.
I'm not an Apple fan and I am not a Windows fan. I believe a personal computer is a tool. I have a mixture of gear at home running on multiple platforms from multiple manufacturers. My next desktop PC is going to be a Mac (95% certain anyway). However for the task in question with the budget the OP has specified I just can't see how anyone can recommend buying a Mac Pro at the moment.
I live in a city full of creatives - I have plenty of friends who work with Macs on a professional basis. I have an interest in tech and I have often asked them why they bought a Mac over a Windows based machine. Most of the time the answer is simply "well, everyone in the industry uses a Mac because they are better". That is a pretty nebulous and incorrect assertion. I don't think Windows machines are better by the way. It really comes down to what you need to do.
If the OP is going to be doing graphics and video work then he needs to look at what software he is going to use. If he has to use Final Cut for work then he needs to get a Mac. If he is going to use Photoshop and Premiere Pro and anything else from the Adobe Creative Suite then they are exactly the same whether running on Windows or OSX. They will perform better on a Windows based PC compared to a Mac Pro and for less money. These apps will utilise CUDA acceleration allowing the GPU you use to do most of the rendering work. Upgrading the GPU in 12 months time will also be much more straightforward in a Windows based machine compared to a Mac Pro. Yes, I know you can upgrade the GPU in a Mac Pro but the choices are far more limited.
Objectively a Windows based machine will perform better for less money and if using Adobe software the experience will be the same.
If general OS usage is going to be the subjective measure then it comes down to personal preference.