Whether the JC/Poly system is more stressful, as what Eugenesmasher said, is dependent on the courses in Poly. JC is already stressful enough, and different Poly courses vary accordingly.
what I feel, is that the JC route would be a lot more pressurising.
Through my two years, I'm always constantly reminded by teachers that the ultimate goal of JC is the A levels. A levels are only good enough to enter University. If you don't do well to go Uni you're gone case. And the list goes on.
And it comes from parents, relatives, etc
Also, when I occasionally visit my friends in poly, I never fail to see bunches of people goofing around, looking very stress free. Pardon me, cause I'm not sure which course they come from or what. But it is at least more pleasing to the eye to look at those people having fun, goofing around, lazing around, then to deal with more than half the student body spending time studying in their free time. That adds on to more pressure for yourself. And you see it everyday in JC. Yikes.