Quick comparison:
Desktop:
http://www.aria.co.uk/Systems/Gamin...70K+4.60GHz+SLI+OC+Gaming+PC+?productId=54541
(From the company I buy my PC shit from, remember you can get even better value by building it yourself)
Mac:
http://store.apple.com/uk/mac/family/imac
Processor: Mac doesn't even state which one . . . you are certainly getting more with the desktop though, top of the range i5 with overclocking + fantastic cooling performance. Desktop simply wins out here every time, OC'ing will make it more viable in the years to come because as programs become more demanding CPU's will have to work harder. You can't OC on a mac therefore will need to upgrade to a whole new PC to catch up with the tech cycle in said years.
Hard drive: 1TB (doesn't even state which model FFS) vs 120GB SSD + 1TB HD (Seagate Barracuda 7200 RPM - good model BTW). Desktop completely outclasses Mac here yet again.
GPU: Mac has a single 512MB mobile GPU which can probably handle average games on medium setting for approx. 30 FPS. Desktop has a multiple GPU setup (SLI) each with 2GB VRAM and they will handle just about anything you can throw at them. Would expect >60 FPS in all but the most demanding games at high resolutions with everything maxed out. Although you might not intend to game with it, gaming is still the best way to benchmark comparisons because it is so demanding.
RAM: 8GB v 8GB. Mac again doesn't state which but you would be hard pressed to find better RAM than that which Corsair (read: best company ever) distributes.
The only thing the Mac has going for it is the screen. No idea how I quality it is but you can get a good deal on some IPS Monitors nowadays. Think I paid £160 for mine. Take away one GPU from the Desktop to cover costs of a monitor, building it yourself should save enough to buy Windows and you still have a better PC. Don't know what is powering the Mac but with a Corsair PSU in the desktop you don't have to worry about anything.
If you don't get it already I'm convincing you not to buy a Mac. Thing with a desktop is if you don't need something (GPU for example) you don't have to have it and can save yourself money. Which can then be used to get better performance in another area, or used to play teh pokerz wif!
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You probably don't get any of that but there is no area in hardware where the Mac wins. Desktop>Mac.