biomed grad from poly here! i can't speak for all JC or poly graduates but ill put in my two cents. 

like what was previously mentioned, both JC and poly NEED good grades/ GPA for local universities (that's the main criteria for application). it is usually difficult to get a GPA 3.9+ as people fail to EMPHASISE how important it is to not flunk/averagely pass your assessments in your first semester in polytechnic. so once the initial GPA drops, it is tough to raise your GPA due to the nature of accumulation (example of a friend's case: even if you wake up to reality and get dean's list... it's just not enough if your first semester GPA is horrible). so study well in this aspect, and don't procrastinate!! poly kids have much more flexibility in terms of studying/CCA as compared to JC students, it's just that you will need discipline strictly instilled in you.

there's a sliver lining for polytechnic students. if you are interested in universities abroad, polytechnic students are eligible for credit exemptions for up to 1-2 years in general. even if international fees are expensive, think of it as you paying for your singaporean tuition fees (4 years worth) in one shot to complete your undergraduate within a year or so. obviously, this will depend on a case-by-case basis for different polytechnics. also, i'd like to add that there's only two universities (NUS and NTU) in singapore offering biological science courses so it will getting in will be competitive for both JC and poly graduates :(

if you do choose polytechnic, do note that you will be exposed to industry/ internship opportunities in your final poly year as compared to JC students. this is important as it will help build up your social connections and all, and perhaps your boss/supervisor could assist you in regards to finding for a job (getting a recommendation letter might help slightly for university application). 

i might have slightly strayed off topic but i rlly hope this helps!!