Personally, I found it difficult to choose between PPE and SPS, mainly because I wasn't sure about the maths, and my teacher really wanted me to do SPS (and I loved Cam).
Have you sat in an lectures? I sat in on an SPS social anthropology lecture, and realised that although it was interesting, it wasn't what I wanted to be learning about for three years.
Obviously, I don't do PPE at Ox, but I feel I must defend it somewhat. I would dispute the claim that the subjects in SPS complement one another more than PPE -Phil is the foundations from which the other two subjects stem, and Economic and Political issues have become inexticably linked in today's world. You can't have a full picture of the EU without understanding the Economic and Political impact of such an IGO!
PPE has the rep (Ox's 'flagship course', arguably) and the subjects covered are more easily converted into jobs, IMO. I considered that future employers would see SPS as the easier option - statistically easier to get into (I know, facetious arguments), and not as academically rigorous.
These are the reasons why I chose PPE (but to be honest, it just boiled down to interest in phil and econ, and a hatred of psychology and social anthropology).
(and you can't walk on the grass at almost all Ox colleges!)