If they are going on to a college, do FSMQ Add Maths. It gives a good intro to AS including some applications and does not tie students to one syllabus. If they go somewhere teaching a different board they may not be able to use their C1 result. Also the FSMQ is a complete qualification which is useful for studentswho choose not totake maths at college. C1 may be more sensible in a school where you will keep them in the sixth form.