I found that neither Physics or Chemistry was useful to any other subjects, rather English Advanced was to the betterment of both. Personally, I loved the Chemistry course and found myself stimulated by the material on a regular basis, both challenging and even at times, fun. Physics on the other hand, despite being a good course to learn and can be very interested with an enthusiastic teacher, is a very laborious subject in respect that its concentration and foundation on historical happenings and their significance are overpowering. I tended to find that the history in the Physics course overbore the actual physics itself and I occasionally found myself bored with it.
That said, Physics is much easier. Chemistry, I spent more time on to achieve the same understanding and level of competence in completing questions because there is (arguably) more work involved in them.
Whichever you choose, I'm sure you'll be satisfied. Better yet, drop neither? I enjoyed 15 units in the preliminary year (accelerated student) and then 13 in the HSC year, so both are very manageable and go hand-in-hand perfectly - there is a certain unspoken symmetry and alliance between the two HSC subjects.