The one with a diploma is certainly better off than one without
any qualifications.
How much you earn will obviously depend on the nature of your
qualification, the demand in the market and the number of
competitors you face. You must consider how much demand there is in
the market for your particular skill and how many people possess
that skill or can acquire it quickly.
If you have a general Arts degree from NUS and decide to work as a
bank officer, you are at a disadvantage. Firstly, your skill is not
specialized enough: the bank can just as well employ a Science
graduate rather than you. If they need more specialized skills,
they would look for an accountancy or finance grad. What do you
have to offer that other grads do not? Secondly, there are many
grads in the market who can compete with you for the bank job you
can do. Thirdly, even if you get the job, it wouldn't be difficult
to replace you with a cheaper, fresh Arts grad 10 years down the
road. Of course, if you are good enough and able to distinguish
yourself in the job, that's fine...just that you have to work that
much harder to keep your job.
Tertiary education is a significant investment of both money and
time. You have to go beyond personal interests and also consider
the direction of the economy. With the IRs coming up, there will be
a demand for people with experience and skills with running a
casino. If I were 17 and considering a tertiary education, I might
consider a course in hospitality management with a special emphasis
on the gaming industry. (For example, here are some links:
http://www.morrisville.edu/academics/Business/Gaming_Casino/student_stars.htm
http://www.easb.edu.sg/updated_pages/uwic_baihm_cm.html).
I would intern myself with various big casinos in Macau and Las
Vegas. When the IRs come online, I would be well-placed to find a
job there and it is well-known that casinos pay well. There will be
few in Singapore who have the kind of specialized skill in casino
management and whoever gets there first will have a better chance
of landing the job.