The fully-BTO-loaded 27" iMac is currently the fastest Mac available by most measures.
I wouldn't recommend a Mac Pro to anyone right now unless they need PCI slots because the current model was first released a couple of years ago. The cost premium for the Mac Pro just isn't worth it for the vast majority of users, including some who think they need the "expandability" or "upgradeability" in the future.
Do some research on the iMacs before diving in.
The aluminum iMacs have been plagued with issues since 2006 and Apple expects you to pay a premium on top of your premium to have confidence that you will have a fully functioning computer for 3 years.
I would not recommend buying unless you can afford the Apple Care plan because this computer will have issues eventually and any issue equals time without the unit since it is an all-in-one device.
Expect to defend yourself and your "work" environment where the machine is kept because Apple tends to blame the harsh users for the fault of their product.
I've fixed many iMacs for people, from several failing HDDs to rebaking solder points to swapping LCDs. Heat is a killer, and these things run hot.
Below you will see comments about operating at normal temperatures..etc, but look up what the maximum temperature any magnetic media should be running at, and then read about how hot these things run. Apple has not implemented special wiring which makes it much more difficult to swap or upgrade a HDD too.
Good luck in your decision.