I did both, ended up dropping both for different reasons.
Biology was interesting to learn about, but you're right. There's a lot more content to memorise. I dropped BIO, but for my prelim exam prep I had about 15k words for 4 topics. But it was pretty fun as I had a good teacher for it. I dropped it as I started becoming lost as we got into digestive systems. Most of the time when you learn about a body part, cell or something there are different parts which is pure rote.
Physics was also great, I ended up dropping it because of the teacher, and plus I wasn't doing as well (even though I topped). Prelim was maths-based, but HSC becomes a bit more content based as it delves more into EM waves. In HSC and a bit of prelim you get an advantage i.e. vectors, and motion-based questions for mathematics.
Why don't you try both and drop which ever one you don't like at the end of the year?