God, no, don't rule out Comberton. It's a very good school, with very little bullying and disruption (and I say this as a parent of a bully-magnet). The intake is probably in their favour, plus the amount of parental input, but they also emphasize social development. If we were in catchment, I'd be as happy for DC3 to go there as to Sawston.
No, they don't require you to take a GCSE in their specialism. Both currently insist on the usual set for most kids: Maths, (double) English, double science, and a language, plus three free choices. Currently Sawston doesn't guarantee you'll get all your choices, whereas CVC pretty much does, as it's larger.
Sawston offers Engineering (and Statistics, oh yum), Comberton has Electronics, if you have a maths/sciencey type of child (can you tell we've just had Options Evening?) Triple science is an option for top sets at both.
Of course, this could all have changed under some new gov initiative long before it becomes relevant to you!
Uniform is cheaper at SVC than CVC, though burgundy-faded-to-pink sweatshirts are not to everyone's taste...