I have both.
Thorn Expedition rear racks on my two Thorn Ravens.
Tubus Cosmo stainless steel rear racks on my old 650B wheeled utility bike, my wife's 650B bike and one of the visitor bikes.
Thorn Expedition and Tubus Cosmo are very good racks, both much more rigid than the Karrimor rack I used for 30 something years (still fitted to the second visitor bike).
Advantages of the Tubus Cosmo - no paint to get scratched. And easily available from my LBS here in France. Thorn is mail order only from SJS cycles
The platform on the Thorn Expedition is, however, much more useful for strapping on stuff-sacks, holdall, tent or whatever. And it seems more solidly built, maybe a tad heavier, but not very significant when adding panniers and the weight of the stuff carried. The system fixing the Thorn rack to the seat stays is cruder - a flat piece of (stainless ?) steel that needs bending and cutting. But I have more confidence in the Thorn system than in the prettier Tubus Cosmo fixing system, which uses an eyebolt that could work loose over time.
My verdict - for looks, the Cosmo, but for practical use I reckon the Thorn has the edge, it will just need the paint touching-up every few years.
The Thorn cosmetic seconds look like a good deal except for one thing - I wouldn't worry about paint defects or blow holes, but a rattle would probably end up annoying me on a bike used for long rides.