I think they're roughly the same, although I did Business at A Level, and economics as a degree,
I have no experience of doing either history or law, so I can't say specifically. However, I am confident that if you put me in a law lesson, I would be either heading for the hills or dropping it like a brick. The main point here is the harder one should be the one you enjoy the least, and the one that fits with your style of thinking the least.
I could do economics and get straight As, but if you put me in a lesson in law I'd probably get straight Us.
Whilst it's not impossible for you to turn things around mid-year and get As instead of Us (plenty have done it before), it's going to be a long winded and tiring trip.
I also take it that you have decided on your university course or the type of job you want to go into, as these choices can affect which A Level you take on, irrespective of how hard they can be.