We used to live in Dulwich and looked at most of the pre-preps there for DS1. We picked DCPS in the end, although have subsequently moved.
Rosemead felt too happy-clappy (not meant to be an insult, although I appreciate it might sound a bit like one) for DS1. DS1's personality is too (parental euphemism alert) big for a school without a lot of space to run around. Rosemead tut-tutted at him not being able to sit still and be quiet when we went for an open day (he probably wasn't even three at that point).
Oakfield was higher on the list - it had quite a nice feel to it (for example, it was very mixed racially which felt right for a school in such a cosmopolitan area).
But, in the end we narrowed it down to DCPS or DUCKS.
DUCKS is quite small. It also has seemed to lose its (effective) guarantee of entry into Dulwich College at a later date, which would have been a big positive for us. Its classrooms felt very cramped.
DCPS is a bigger, pushier school - a lot of children from there do go on to Dulwich College but a lot also go on to other, very well-known schools - which can be a positive or a negative depending on your outlook. Our neighbours at the time sent one of their children to Tonbridge after DCPS; the pupil who escorted us on the open day was going to Marlborough at the end of that year.
For us, it was the greater apparent flexibility of DCPS (i.e. not so committed to a senior school) that was a key factor. Also, the first two years (Nursery and Reception) are mixed sex and at a separate site (by Belair Park) which felt like a nice introduction for those first two years.
Bear in mind that DCPS can be quite hard to get into. At somewhere between age 2.5 and 3.0, DS1 had an assessment (without parents present) following which he was offered a place. But I understand only about 1 in 3 children who had the assessment who were offered a place.