Really there are no definitive answers for what to do but personally... I would go with Computer Science out of those two. It has a much larger scope for employability and the like so you're more likely to find a job - however I am just a person on TSR and nothing will beat pure research into both fields.
If I was you though, also consider Engineering courses if your strong subjects are Programming (Computing), Physics and Maths. They all go hand in hand for quite a few Engineering degrees so just look around. Those are good subjects and assuming you have sufficient qualifications (i.e. GCE) in them, they can take you far.