Being a photographer myself, I can only address this side of your decision. Back in late summer 2013 I was facing the same issue about a new MacPro to replace my 2008 dual core 3,1 and solutions for long term photo storage. I went with a 5,1 hex 3.33 GHz core MacPro. I loaded it up with at first 20 GB and more recently 32 GB of ram, a 3GB 7950 graphics card, and it absolutely flies with large PS files, filters, layers and plug-in editing software by Nik and Portrait Professional. I have dual 27” Apple displays and am very happy I got the 6 core.
The one thing you haven’t discussed is storage. Being a photographer my most import photographic equipment is my fast prime lenses. Second most is secure digital storage and lots of it. This is why I chose the 5,1 over the nMP, because of the internal storage and other PCIe based options it offers.
Just as some “food for thought”, here is how my 5,1 is now set up; 480 GB partitioned SanDisk SSD in lower optical bay with OSX and Windows boot drive. 2 HGST Enterprise class 4TB HDD in bay 1 & 2. These are set up as RAID 0 for user files and photo storage in a 6TB partition. Each drive has at the outer layer a 1TB partition that I use as a (very fast) scratch drive for PS on one and a backup to the SSD startup drive on the other. Bay 3 & 4 are 2 enterprise HDD at 3GB each, set up as 6TB RAID 0, to be a direct, backed up twice a day copy of the Main RAID. So the base MacPro tower, without any external drives offers me 10 second boot times off the SSD, 6TB of data files and 6TB of backup at 312 to 320 MB/s Read Write speeds. I doubt you will get these speeds with Thunderbolt… Plus I have 2 other eSata dual drive RAID 0 enclosures that I back up weekly, one goes in the fireproof gun safe the other goes out the door with me on any photoshoots or trips. These drives back up via a PCIe eSata card at about 240 to 280MB/s speed. And if I wanted I still have the option to add a faster 6Gb/s PCIe card that could house 2 additional SSD internal drives on board and thru external ports, most likely speed up the 2 external drive transfers as well. ESata is a much better option for ME than Thunderbolt will ever be, and is a lot less $$.
My total cost for 4 RAID setups and drives, 2 internal and 2 external was about $1,200. For comparison, 6TB Thunderbolt enclosures are going in the $600 range and you would need 4 of these to duplicate my setup. And if interested, you should be able to locate a 5,1 hex core for <$2,000.
The other important reason for buying a 5,1 is Firewire 800, it is long from dead IMHO. My wife and I are a 5 Mac household (Her 27” IMac, 2 MBP 15” laptops and MacMini for iTunes and Apple TV) and all of our Macs have FW800. So moving data around is easy via a 2TB My Passport FW800 drive, lot faster then wireless. On another 1TB FW800 portable drive I have partioned OSX Snow Leopard and Mavericks systems with utilities to act as the startup drive via the target disk mode if any Mac needs a tune-up or has problems. I keep a copy of each Macs HD recovery software on the drive just in case. The target disk mode is a life saver and it is a real shame Apple did not implement it with Thunderbolt.
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