I recently built my own photo editing PC and then helped my nephew build one for himself.
I think you can build a PC that will do what you want. You may even discover that you need two. But it just depends on your needs. Do you need mobility or does a workstation make more sense? Do you have the time and patience for a DIY computer build project? Its not exactly difficult and shouldn't take more than 8 hrs. start to finish.
Two problems with the iMac and the MacBook Pro or laptop. Heat and upgrade path. Think long-term use.
Heat is a computers worst enemy.
With a desktop/workstation, its much easier to upgrade or expand when you have the cash and time.
If you decide to go Mac, go with a Mac Pro. You might decide you need and/or like the flexibility and mobility a laptop gives you. Work on the go, final edits on Mac Pro ... all comes down on how you want to work.
Here's a DIY option to the Mac Pro: http://www.motherboards.org/category/877/mac-killer
Use Newegg, Microcenter and Amazon to source parts. I like using Newegg because you can place items in a cart as a list with a name for example: BuildOne and save it latter for comparison or part substitution. This way, you can source the parts on Newegg, and compare the costs of different configurations.
After about a month of researching and comparing costs, I believe I spent less than $1700 last year on my particular build. Granted I caught a few sales but it just goes to show that you don't have to spend $3500 unless you really want to.Motherboard
: ASUS Maximus Gene V microATXCPU
: Intel Core i5 3570K 3.4GhzCPU Cooler
: Cooler Master Hyper 212 EVOMemory
: 16 GB G.Skill Ripjaws X series DDR3GPU
: EVGA GeForce GTX 570Boot Drive
: Samsung 840 series 256GB SSDHDD1
: Western Digital Black 2 TB 7200 rpmHDD2
: Western Digital Green 2 TB IntellipowerOptical Drive
: ASUS 24x DVD burnerPower Supply
: Seasonic X650 GoldCase
: Silverstone Temjin TJ04-E (comes standard with 3 case fans)
I went with a microATX gaming board and a mid tower case for features, air flow and ease of cable management. The mobo will handle dual GPU's to handle video demands and supports 32GB of memory.
The case isn't exactly a tool free case but its USB 3.0 compatible and I liked it. I may decide to add another SSD as a scratch disk for Photoshop and/or LR4.
If you feel comfortable DIY, I say go it. For video editing, I'd go with the i7 3770K and 32GB of memory.
Good luck and have fun.