Well, Harry, in my humble opinion your choice would depend not only on the weight of the rider but on the use to which the bike is to be put ie road r acing, time-trialling or just fast training or pleasure rides etc.
The CX 18 to give it its full name is a very narrow section rim designed al most specifically fror time-trialling using narrow tubs of less than 20mm s ection..mostly 19mm, whereas the SSC is a rim designed for tough road racin g purposes ie Paris-Roubaix at the extreme using 23.24/25 mm tubs.
The CX has a width of only 18mm and weighs 300gms, whereas the SSC, in its Blue of silver form was 20mm in width and weighed 395 gms, and the grey SSC - the top rim -was 21.5mm wide and weighed 400 gms.
I think that these stats say it all.
Norris Lockley,,, Settle UK