Personally my view on that would be that they are trained for quite different roles. the SAS are far more likely to have to spend long periods surviving and observing behind enemy lines than Delta (who seem to be more of specialised "hit squad"). and as such their skills are going to be in different areas.
US Special forces tend to be more specialised than the SAS who have a staggering number of roles. Combining the roles of units such as Marine Force Recon, Special Force ("Green Berets"), Delta etc. in a unit that is a fraction of the size of any of them. It isn't really a case of who is better given that you are not comparing like with like.
The SAS and SBS also have the advantage of a unparalelled number of active operations (Borneo, Malaya, Aden, Radfan, Oman, Vietnam, Northern Ireland, Kenya, Somalia, Falklands ,Iraq, Yugoslavia, Sierra Leone, Afgahistan and im sure many that we haven't heard about) that have all contributed to the skills pool that the SAS possess. The fact that US Special Forces "Fell out of fashion" after vietnam means that lost a lot of that experience and those skills.