take the Jstar
Cathay is a company which is still recruiting on its reputation of the old A scales. Cathay is living on its reputation as a Legacy airline. The truth is that its very close to already being a Low Cost Carrier. At least Jetstar isn't pretending to be a career airline from the 1980's.
Both companies are equally as nasty at times.
If you are going to live on a lower wage it might as well be in Singapore rather than the pollution of Hong Kong.
And most importantly a FO position means that if you don't like it, you can apply for jobs in other parts of the world. A SO position means that if you don't like it you can suck it up, complain and not do much else. Your SO time is close to worthless on a CV.