My sources tell me that the Intensity Extreme should be shipping VERY soon. Officially it's supposed to be out Q1, but it may start shipping as early as this month.
The Intensity Shuttle Thunderbolt will be later, probably more like early Q2.
As I had surmised, the Intensity Extreme is essentially a more rugged (aircraft grade aluminum casing), portable, and Thunderbolt version of the Intensity Pro [$189] card, while the Intensity Shuttle Thunderbolt is the desktop version. One downside is the Intensity Extreme doesn’t do 1080p60/50.
The Intensity Shuttle Thunderbolt (the original Intensity Shuttle [$189] is a USB 3.0 product) will do up to 1080p60/50, but it's larger, not as rugged, and you'll have to wait for it.
Blackmagic tells me that customers that ordered the Intensity Extreme when it was announced, then realized the Intensity Shuttle Thunderbolt is better suited to their needs, can change existing orders that have not been filled.
Finally, apart from the 1080p60/50 difference, technically, there are no image quality differences between the two. Quality would be effected only by variables like source, cables, etc. If you were to have the same inputs/outputs for each, image quality would be the same.
Blackmagic Design: Intensity Models