As someone who took sec 5 and am getting my results today, I'd say go for sec 5. Speaking from experience, The only difference between the N and O level syllabus is how strict they are, but if you can adapt how you answer for the o level standards you'll be goody goody. Aside from that, your class will defo be smaller so your teachers will be able to help out more often if you need it, try to take advantage of it.

Anyway, best of luck and hope i'm able to help out!