Law is very heavily writing based. Year 12 is alright with mostly 10 markers coupled in with a few "front end" 3 and 5 markers (such as "describe the aims of sentencing" or "describe the procedures to trial at a Magistrates court") where your 10 markers are your juicy ones on an offence - for example you might get a 10 marker on section 18 grievous bodily harm. These questions are all incredibly easy the second you learn how to structure and learn the cases (you need to learn cases to demonstrate points, e.g. R v Ireland 1997 for Assault), but Year 12 is an incredibly easy year in Law, especially Law 1 which is just learning how Laws are made in Parliament and whatnot.
If you decide to continue it into Year 13, Law 3 is basically Law 2 (the 10 markers) and then some, where the entire Law 3 exam is just 25 mark questions. You do the offence (usually 2 offences with 1 defence etc) like before, but you also need to learn defences (like intoxication, insanity, automatism, self defence, prevention of crime, etc, then the special defences to murder which are diminished responsibility and loss of control (((you dont learn survivor of a suicide pact)))). And yes you do murder, voluntary manslaughter and involuntary manslaughter - which are very interesting topics. Law 4 which we've just started is stuff like theft, robbery, blackmail, etc. You also have to evaluate the Law in Law 3 which is quite similar to YD to me (for example if someone made a thread on an issue with a Law), for example an issue with Insanity is sleep walking counts as insanity so if you were sleep walking and knocked someone over you could get "not guilty by reason of insanity" and the social stigma behind that is horrendous, etc for the other defences, so it's a very interesting topic
Basically it's a very interesting subject but it requires a lot of writing and a lot of theory - most things come in groups of 3 to learn (like the McNaughton rules, causation, negligence, etc).
As for maths I can't really comment on it but over the Christmas holidays they got set 27 past papers to do - all full exams - but their teacher is a bit of a cock (but a cock who cares), but they basically just practise practise and practise as imo that's the best way to learn a subject like maths
Personally I'd recommend Law but that's my bias so just go to sixth form open evenings and go to Law and Maths to see what the subject is really like and then make a decision based on that