by the sound of what Chris is saying, it looks like Apple is trying to lock in its uers once again.
What is best for you may be very different to me, but the hacking order is, Macpro, iMac, Macbook Pro, Macbook, MacMini, Mac Air and iPad, iPhone/iTouch/IPod with the later being the lowest spec device.
The Macpro is a workstation, a much more heavy duty machine then the iMac. The Macpro can have dual processors, more ram and more hard drives and you can add PCIe cards to it to expand and add functionality to it.
The iMac is similar to your windows desktop computer range. Its single processor and can have up to two hard drives (2.5") i think. up to 16GB of ram on the latest ones. I doubt you can replace and upgrade the graphics card in this one, which may be desirable if you are doing HD Video.
At the end of the day, I guess you need to look at how much you are willing to spend vs how long you are willing to wait for things to be processed/rendered.
Looking at the specs of the new iMac's they look very nicely spec'ed but so is my windows workstation :-)
All the best and let us know what you decide to get and why.
PS: If I were to get a Mac coming from the windows world of hot rods, It would have to be a Macpro, purely because it represent the most powerful machine in MacLand with the most option for future upgrades. So as an investment, I think the Macpro is a better choice. They just don't build them fast enough :-)