RE: CPU speeds...
Hi crashwins,
Okay, so you already have an external Apple display, so this satisfies the screen "real estate" criterion, which in my experience is of major importance.
Then I'd have to agree with the others: if you are doing photo and video editing, your next most important consideration is CPU/GPU speeds. Some of these editing programs can utilize the GPU, some can't, some of these programs run well in parallel, others do not. Most of them can use more work on their parallelism, and I suspect over time all of them will get faster through better parallelization and greater GPU (openGL) utilization.
Using current editing programs, I think everyone that does this type of work finds that the bottleneck resides with the speed of the CPUs. Rendering, modeling, 3D motion, textures, lighting, just take an immense amount of CPU time. We all wait on the CPUs to finish their rendering tasks.
Given this state, then obviously you need the fastest CPU/GPU combinations. If your work is really intensely and nearly exclusively photo/video/motion/rendering editing, then the fastest CPU/GPUs make the most sense. Now with the new Ivy Bridge processors and the fact that the quad-core i7s have been included in the Mac minis, I actually like the idea of getting one or two Mac minis to act as rendering servers, and then perhaps an Air for when you require portability. You won't be giving up much in terms of CPU speeds for the Mac minis over either a rMBP or an iMac, although of course you will be giving up the fast GPUs in the rMBP and iMac.
Of course, it also goes without saying that you must have large amounts of RAM for this type of work. Currently, the MBPs allow for 16GB while the iMacs allow for 32GB. If you are doing video editing, the more of the video that resides in RAM the faster you will be able to edit it. SSDs and Fusion Drives partially alleviate this requirement, as the SSD/FUD is roughly 4-6 times faster than a standard HDD.
So, to summarize:
(1) iMac: Fastest GPU, fastest CPU (quad), largest RAM, FUD, reasonable price
(2) rMBP/uMBP (+your Apple Display): Faster GPU, fast CPU (quad), good RAM, SSD, expensive
(3) Mac mini (+your AD): Fast CPU (quad), okay RAM, FUD, reasonable price
(4) 13" uMBP/MBA (+your AD): Slower CPU (dual), okay RAM, SSD, reasonable price
In my opinion, option (1) is the best in terms of the capabilities you need for the least amount of money; it supplies the best bang for the buck even though you aren't using your Apple Display (maybe later you could add a Mac mini to your system to both utilize your AD and act as a rendering server). Unfortunately the FUD adds a lot to the prices, but I feel that for photo/video editing having either a SSD or a FUD will be important to your throughput.
Good luck,
Switon