Bob and Chris, Thanks a lot for your answers. A Lacie 256GB Rugged USB 3.0 / Thunderbolt SSD is on my list now to edit my pictures there until Thunderbolt 2 price drop.
Well, if you are not going to be using any multiple disk thunderbolt enclosure or external PCI Express thurderbolt disk, the advantage of an SSD disk with thunderbolt vs USB3 is minimal if any.
You need to buy and USB3 enclosure that supports UASP protocol (http://www.tomshardware.com/reviews/usb-3-uas-turbo,3215-4.html
), and you will get around 400 MB/sec read and write to the disk ( http://forums.macrumors.com/showthread.php?t=1399775
), major drawback, no TRIM support. The Inateck enclure cost less that 20 dollars in Amazon, plus and Samsung EVO or Crucial MX100 SSD drive and you will get those speeds (makes no sense to go for top of the line SSD, you are not going to see better speeds like 500MB/sec to 600 MB/sec that it is the top of SATA II). For 200$ you will get a 512 MB, it is not rugged but since you are connecting it to a Mac Pro I'm assuming that you will not be moving it around and you get same speed or bit higher for half of the price http://www.pcmag.com/article2/0,2817,2460576,00.asp
Again, unless you go multiple disk or some PCI Express over thunderbolt solution, you just paid more for the thunderbolt controller. Multiple disk solutions will allow you to go over the 600 MB/sec barrier (if you use SSDs... )
P.D.: Beware that not all USB3 Hubs like for example the Thunderbolt of Belkin that has an usb 3 hub on it, some of them not allow or UASP protocol or maximum USB 3 bandwith. Belkin caps the speed of USB 3 to 2.5Gbps instead of the 5Gbps of the USB3 ports in your Mac Pro.