depends on how much you can improve on chemistry or chinese heritage
a high band 4 chemistry is only slightly lower than a high band 5 chinese (continuers?)
a minimum band 5 chemistry is higher than high band 5 chinese
if you can improve on slightly chemistry and still only maintain your marks as high band 5 chinese, then chemistry is generally more worth it
it also depends which one you're more interested in, and which one you may need for university i.e. future courses