Hi Andrea,
Thought it'd be easier to clarify your doubts here instead.
Well like many other institutions in Singapore, having good grades is the key to everything, same for doing a degree in local universities. Based on last year's admission grades, I'd safely say that having a GPA of 3.5 and above would be quite safe to enter a local university.
That does not guarantee that you'll be able to enroll into any course of your choice because certain diplomas only allow you to choose from a limited number of degree courses but you'll get some module exemptions.
My advice is really to sit down and plan before making up your mind on your choice of tertiary institution. Truth is, the competition in Singapore is way too tough compared to other countries.
But if you ask me, I feel that the safer/easier route to university would still be the JCs. Apparently, more seats in each course are allocated to them. Oh and please don't get the misconception that Poly is the 'slacker' choice because it's not, the projects can drive you crazehhhh. Trust me, I know. Poly/JC, you'll just have to work hard to get the course you want in university, especially so when now there are so many 9 pointers going the poly route which means even more competition!
Yup, that's my take. I do hope I was of some help :)
P.S. I've never regretted choosing the poly route even though I could get into a JC.
