Cat bridgedeck hieght
That is the most absurb thing that I have ever heard. First of all there are a fairly large number of builder in both country of cat''s and their designs vary so widely that there is no standard comment that you can make about boats from either country. Second, if either country built cat''s designed to deal with rough weather it would be South Africa where they routinely sail in some of the roughest waters that the world has to offer. (If you have any questions about that pull out an atlas and look at where Capetown is located.) On the other hand the Bay of Biscay no walk in the park.
As to weight, both countries offer both high performance lighter weight cats and also heavy weight cruiser. If I had to guess in a broad general sense, the South African boat would be better engineered but that could easily vary on a model by model basis.
Lastly in 6 foot short seas, the pounding on almost any cruising cat can be unbearable.