DD attends the lower school of SPGS, while her close cousin is in the same year at LU. We often compare their experiences indirectly.
Both DD and her cousin enjoy their schools very much. They have a range of subjects and clubs to choose from, and both schools keep them engaged and challenged academically. The exams and workload are comparable for both schools in most respects so far.
There are some high-flying parents in DD's year, and a few of them stand out with extreme high expectations, but this has nothing to do with the school. We believe such parents are present in LU as well.
Regarding diversity, more than half of the girls in the lower school of SPGS have Asian backgrounds - DD is also Asian and she thinks the school is fairly diverse culturally. LU has more ethnic diversity and I'm not sure if that matters.
In terms of social, economic, and geographic factors, I would say most of them are from the comfortable West London families, though there are also many from the NW/SW suburbs. DD and her friends feel fortunate and privileged, but not entitled.
It is true that the majority of the students in her year come from private junior schools. LU probably has more students from state schools, but there are also many from Latymer Prep.
As PP said, it is hard to say which schools are more suitable, since every school is different. And even the same child/parents in the same year can have a different experience. The most noticeable difference between the schools is that one is coed and the other is a girls school, and that would be my main consideration personally.