What makes you think so? As others have pointed out, if you
don't get into Uni, you are definitely worse off. A JC leaving cert
is not worth the paper it's written on. A poly diploma will land
you a job.
If you do well in the system of the teaching in poly and do well
there, you can get into uni after that. Poly grads who do well
enough to get into NTU regularly top the class in Uni.
Finally, between a fresh poly grad and a fresh uni grad, I would
rather employ a poly grade and train him/her up. If I like the
worker, I would rather pay a poly grad the pay of a Uni grad. He
will be grateful for the higher pay and is more likely to stay with
me. If I employ a Uni grad for the same salary, he will be tempted
if someone offers him a hundred dollars more.
Go to JC only if you are sure you can get into Uni and you have in
mind a professional job that the poly grad cannot compete
in.