(speaking of personal experience- edit: whys said it better in the post below me!
)
The content heaviness of the subject really depends on your school, but generally, both subjects have a substantial amount of content. In psychology, you learn content blocks one at a time so you'd focus on one area before moving onto the next. An example of this would be- We learnt about the parts of the brain at the start of Unit 1, then moved on to a research task, and then we moved onto developmental psychology. So basically, it isn't really light stuff you're learning about, but it's a bit easier to sanction off the content you're learning at a time.
In HHD, things like definitions and terminology (and especially in the way you answer questions) blend into each "content unit" so you'd need to remember those to relate them to the new things you're learning.
To give you a better understanding of what I'm trying to say, it'd be like this:
You'd begin HHD with learning about the dimensions of health and wellbeing (physical, emotional, social, mental and spiritual.) Then, in a later unit, you'd learn about Medicare. In a SAC question, you might be asked for example "How can the services provided by Medicare improve physical health and wellbeing?" I hope this gives a better view of what I mean by things blending in HHD.
I wouldn't be discouraged that they're heavy subjects. If you're doubting doing Psychology and pick HHD you might find that HHD has a lot of content too, you know, it's subjective honestly. That being said, if you aren't too sure about psychology, I think you'd like HHD more. It's less science based, so it won't be too similar in workload to Biology and Chemistry. You really learn how to answer questions in ways where you address different points all together (seen in the above example.) As you mentioned you want to get into medicine, it'd be great that you're learning about the details of the Australian health system and gaining insight into how healthcare affects different people in Australia. If you'd like any excerpts of student work or information about HHD I'd be more than glad to help you out. (: