Think of it this way: you will never regret your decision if you get a Mac Pro. You might regret your decision if you get an iMac. You run a far greater risk of outgrowing an iMac than a Mac Pro. If you’re building a business around your system, go with the Mac Pro.
The Mac Pro has far superior expandability and supports far more options for external monitoring and storage than any iMac. Make sure you choose the ATI Radeon HD 4870 GPU for best performance in Motion and Color.
Storage
Skip Firewire 800. Instead, consider these options:
– Software RAID-0 using two or three additional internal drives. Drive 1 should be a dedicated system drive. If you go this route, make sure you back up the contents of your RAID volume regularly, as RAID-0 has no redundancy and loss of one drive will result in total data loss.
– External RAID of some sort (eSATA, E-PCIe, SAS) – there are many great options out there. Check out the CalDigit HDElement for a good low-cost RAID-5 solution.
Video Monitoring
Invest in a video I/O card and a video monitor (or a consumer TV at the least) for proper monitoring. You can’t depend on a computer display for an accurate representation of how your video will look. An entry level option would be the Blackmagic Intensity Pro. A better (but pricier) option would be the AJA Kona LHi.