hi there, welcome to the forum!
I have had a son at Judd and at TWGSB and I havent noticed any difference in the expectations of work in both schools.
Its not in the schools interest to see boys 'struggle' or fall behind so I have never noticed or heard of friends children getting 'left behind' in either school. My feeling at TWGSB is that they are trying hard to be like the SS and expect as much from the boys as the SS do.
My own opinion as far as you are concerned must be however how you feel about the school for your son. Did you get a good feel at Skinners? Did the teachers talk to your son -how did he find it? It may sound obvious but you are right not to assume just because he has got the marks for Skinners its right to send him there - it is more about what the school offers your son. What about sport? I gather Skinners is bigger on sport (epecially rugby - all boys do it most of the year I think, but at TWGSB they do it on a carousel basis).
Also clubs and subjects at GCSE- really worth looking at. TW offers things like resistant materials and graphics - if you think this is something your son might chose this might be something to consider.