I don't know anyone on this forum personally so I can't say any of us are that qualified to provide you with advice on the graphics card choice, the simple answer is the PC will have more options in the way of graphics cards.
My advice stops at 2D but at CGTalk (You may also find links to specialist 3D websites via CGSociety or CGTalk) they should be able to give you advice on the cards available in the mac pro, various options you could have a think about. One thing would be despite the ample room in the mac pros case, can it do everything you want/will want in the future?
One of the great advantages of the mac pro is how many monitors you can run if you have the video cards. Just think about the dropping price of video cards! Its amazing to think that in a few years time you could be running 7 monitors, no problem! (I don't know if 7 is the limit, its just a nice config)
With the mac pro you may find you can get more work done, if you are working with something like a wacom graphics tablet you can zip around and be working in your favourite application in no time at all.
I'm yet to be amazed with the simplicity of Mac OS X, I would be frustrated if it did anything else.. Its nothing amazing but it has the potential to save a lot of time
You will have extremly fast boot up times and everything you need easily reachable. Does the OS offer everything you want to do, or may want to do? Its brilliant if it does, i'm yet to find an operating system to support all the programs i'd like to run
If you want to play games, theres no real way around it, its on a PC, sure you can play games on a mac but windows is the major player, I won't go into using Boot Camp to play games because, in all honesty, i'd rather use a dedicated windows machine.
Your problem isn't as simple as someone just wanting a hastle free computer to do some photo-retouch on and thats what makes it so interesting!
Whatever platform you choose, for the power you are looking to buy, anything will be enjoyable