If you don't like the teacher, you wont enjoy the subject and you'll loose a lot of motivation throughout the course. If you're enjoying and doing really well in D&T, I think that'll be the better option for you. Its major counts for 60% of your HSC Mark for the subject, unlike Chem, the stress is spread across the year rather then being focused into a couple months of built up stress towards the HSC. You get rewarded with a 1h 30min test compared to a long 3h test. That's just my two cents on the topic.