Modern - you learn a lot of 19th-20th century related history, which can be very interesting if you like story telling. Analysis on the other hand isn't so much.
Legal - you learn a lot of stuff that actually affects your society, legislations, stuff related to crime, as well as applying common sense to situations
Imo I did both modern history and legal studies - I found modern to be a lot more difficult because you had source analysis (which for my school was marked harshly), as well as having to memorize a crap tonne of facts and figures, and then having to construct an essay out of it. You were pretty much fucked if you studied one thing and then got a different question to what you were studying, simply because the facts and stuff are all very different. That said, modern history was still interesting despite having a crappy teacher.
On the other hand in legal studies - I studied 4 topics, but that pretty much felt like 2 topics. I did Crime - International crime, and then linked it to World Order. Then I did family law and that linked very well to human rights. The crime topic relates well to human rights. I didn't actually end up studying for world order because I used stuff from international crime to answer my essay question. Literally legal studies has very well linked topics. A lot of the legislation/cases can actually be used not only in one, but two or even three topics - probably meaning a lot less to remember
Plus legal studies is easy to BS if you tell me. Sure it doesn't scale as well as modern but imo it's a lot less work than required.