History Extension has A LOT of content you are expected to know. As with English Extension 2 it requires a lot of discussion but the main difference is that in EE2, your whole course is moulded around your 1 submit. While in History Extension, while you do have a very important internal project (80% of your mark), you still need to know how to execute essays since both your HSC and Trials will have these essays.
If your more suited to reading a lot of fiction, and are a bit more on the creative side, then you should do EE2.
If you are able to do a lot of quality research, as well as having the dedication to do all the coursework and learning the essay technique, then you should do History Extension
In the end, it comes down to your preference. I recommend you look at both the EE2 and the History Extension syllabi as well as asking the teachers for those subjects for advice before you make a final decision
Hope that helps!