I did both subjects and came first in my year for Modern History
Personally I found Economics quite interesting especially from Year 11 to Term 1 of 2012. However, our teacher retired after Term 1 of Year 12 and we got a replacement. She was good but wasn't quite interesting or made the subject as interesting as before. But nonetheless the subject itself may or may not interest you, I did find how certain aspects affects the economy pretty interesting though.
Modern History in my opinion was always interesting, as we did Russia, Leon Trotsky (Personality) WW2. They were quite interesting topics, but not everyone does the same topics. The only compulsory topic everyone does for Modern History is WW1. And for the HSC exam WW1 is also used to judge in terms of Essays from different topics. For example say Russian Question in the HSC has lower marks than the German Question. They would then look at the WW1 to judge whether the Russian Essay was harder than the German one, which would led to them adjusting the marks.
But interesting subjects both of them. If you're good at writing Modern should be good for you, and if you are interested how some changes or even big changes affects the economy as a whole it should also be good for you.
Best regards.