Errr who is making the judgement that SPS is easier than PPE? Has anyone compared the exams? The marking procedures? Level of coursework?
The two subject share one compulsory topic in common: Politics
From what I understand it is very different politics at both universities. The only thing that is obviously less of a requirement at Cambridge is the ability to do Maths. Personally I think that doing statistics should be a compulsory part of the course and those doing sociology and psychology should do a lot.
The reason why people see PPE as more prestigious is that it has existed for a lot longer. the SPS/PPS tripos is about 25 years old so there are relatively few alumni (the vast majority of whom are too young to have become MPs, CEOs, cabinet ministers etc). I have no doubt that over the next 10-15 years that will change