Whether Poly or JC, this is a matter you have to decide (and
take self-responsibility) for yourself. There are pros and cons,
and no easy right or wrong choice.
Regarding Combined Science versus Pure Science, although the
obvious con is that if you don't do well in your combined science,
most JCs won't let you take H2 Sciences (and H1 Sciences are
useless for use as prerequisites in Uni science course admission
criteria).
There is a pro, however, to taking combined science. The bell
curve is much less steep for combined science, which means it's a
LOT easier to score a A1 for combined science. And once you get
that A1 for combined science, most JCs will let you take H2
Science.
As to combined science being a disadvantage in terms of catching
up, not much so. Both Combined Science and Pure Science at O levels
is almost equally kids play compared to H2 Science
(eg. combined science is level 1 difficulty, pure science is level
2 difficuty, and H2 Science is level 10 difficulty), which means
whether you're from combined science or pure science, you'll have
to start from scratch for H2 Science. So don't worry, taking
combined science is really not much of
a disadvantage.