I did the Reigate one last year (my first 10k) and it was a really hot evening, with a hard hill. Much steeper than Croydon, which I think is mostly the Lloyd Park area? But the run down into Priory Park after the hill is lovely.
Anyway, I'm booked in for Reigate so would be nice if any other forumites can make it. I don't think the numbers are that great, only a few hundred runners. I'm mnumber 8 and only booked in weekend before last, so probably plenty spaces left!