Hey, I sat Higher Biology last year and thoroughly enjoyed the course (got an A!)-
There are a couple of topics which are in both biology and human biology, such as respiration, mutations, gene expression etc. (more general biological processes), however I would just say it is down to your own personal interests and preferences. Biology goes more into depth with ecology (eg. social behaviour, biodiversity, photosynthesis) as well as metabolism, whilst human biology focuses heavily on areas such as the heart and brain as well as reproduction. Ultimately, neither course is 'better' than the other, and your final grade will be equally as valuable in either of them, it's just down to what you would find most interesting to learn about. On BBC Bitesize you can check out the topic lists for both courses so you can see what you think. Honestly, I would recommend either, and there is no need to panic- it's a great subject to consider!
Hope this helps