Modern is considered to 'scale' better but I don't think that's actually true and if it is true it would be a very small difference. I was considering doing both for my HSC but decided to do 1 so I could do another subject and even just modern is bit too much sometimes because if you get unlucky with a topic you don't care about + the class taking ages to get through the work it can be very boring.
From doing modern the good thing is that if you can write well you can always get a decent mark - my worst essay of the Y12 year was still a B6. The common core topic is interesting (to me) and the rest depends on your teacher. If you know what topics your class would be doing you could say what they are and maybe I did some of them too.