Vik,
The Thorn rack is about 13cm wide and about 38cm long in the top load platform with 4 welded/brazed cross supports plus 1 each end bolted. There is a triangulation effect, with a top width of 13cm and a dropout width of about 16.5cms. The welds/brazing looks a lot more sturdy on the Thorn and the tubing sound thicker when tapped.
The Tubus Cargo has a top width of about 12cm and a length of about 31cm with 3 welded/brazed cross supports. The dropout width is about 16.5cms for the 2 support legs, one of which is obviously triangulated. The third (rear) support is tacked on the outside at the dropouts but does not obviously add anything to the triangulation effect, being more of a pannier sway preventer for floppy panniers.
In use both racks are very stiff in all directions, there is no visible deflection in use under load. I tour with some sons and have been able to watch the racks in action. The maximum load any of us have carried on the rear is 20kgs, so not tested to any extreme yet. I have a tall son who uses one of the Thorn rear racks and he has failed to find a fault with it. His riding style will have stressed it, his size 13/14 feet mean that the panniers are as far back as they will go and add leverage when heavily loaded. I doubt he could ride with the Tubus Cargo, he would have to use a Logo and some rear extenders.
You have not said which panniers you plan to use. The back support and clip system may be a consideration in rack choice. I have retrofitted Arkel cam hooks to my Carradice Super Cs and am delighted with them.
In summary, you cannot go wrong with either rack for fully loaded touring, but only you can decide which features best meet your requirements.
Julian