I find I practically impossible to accurately calibrate the Imac screens, so I would take the Mac Pro and connect a good (non Mac) photo & video editing monitor to it. If color accuracy is of importance in your workflow of course. For example a monitor from Eizo, Nec, HP, Samsung, or Dell. I work on both an Imac and a Windows workstation. But for color accuracy in my workflow I definitely do not want the Imac screen (but I do really love the Imacs, they are such a beautiful pleasure to work with!) Or you could of course hook up a second calibratable monitor to the Imac, a solution which I also have used. But if all your clients already have been accepting your work as you made it on your 30" Cinema Display, then you already doing great of course.....
One other advantage the Mac Pro has over the Imac is that it has HDMI, the almost universal monitor / HD TV port. afaik Apple again on this latest new 5K Imac did not add HDMI, but these ports are there finally now on the Mac Pro, because so many Mac Pro users have been complaining that they need more connectivity / compatibility for their workstations. I feel it is a little silly that my Imac is the only device in my house ( and I have a lot of devices) that I cannot easily hook up to other screens because Apple each time refuses to implement main stream ports like HDMI. Even my smartphone has this....
You say you are replacing a Macbook Pro = a laptop, but now you are basically replacing it with desktop versions, the Imac or the Mac Pro. So before you did all this graphics work including video editing on a laptop (Macbook Pro) with only one harddisk? Or what is the Thunderbolt storage system that you have and how do you use it?
Good Luck!