modern is more interesting but harder, because it is so recent, so u need to know loads of accurate info. because it's harder it scales higher than ancient though.
for ancient u can make up crap and just say stuff like, 'an unknown source said' and 'it is argued that.... but we don't know for sure that..' and all that crap, because it was so long ago. ancient is generally a lot easier because of this, but it has a heck of content you'll have to get through- same goes for modern, but i think that ancient has more work.
i personally would go with modern- if ur into wars, dictatorships and political, social and economic crisis stuff then thats for you.
but if u like the aztecs, mummys, spartans, troy, minotaurs and all that ladida crap then you should go with ancient. mind you, ancient can get quite boring. i didn't really enjoy ancient as much as i did modern, yet i did heaps better in it. i dunno- whatever floats your boat, i guess.