I'm a student myself, so I have no idea whether employers look
for 'O' level certificates in job applications, but here's what I
think:
The diploma is more valuable than the O level cert in this
case.
Unless you want to get an 'A' level cert (JC standard, very
tough) and proceed on to get a university degree (at least 10 years
from your estimate given and a lot of $$ needed), you're better off
with a diploma. Diploma is something like going to poly, and you
can skip the O level part too (5 years is crazy, that's a hell load
of time gone from your life).
Pick the course you want for the diploma wisely, though. Make
sure you're truly interested in it and that there is market to
accept you in future.