I think if I were to do straight commerce I'd have a substantially lighter workload in 1st and 2nd years. Some early law subjects, such as contracts and torts, required a bit of reading. Criminal law was a joke. I did no reading and got straight HDs.
I find commerce very easy. In my first year, I skipped nearly all my lectures and attended around 80% of my tutes and got HDs or high Ds. All you have to do is memorise a few formulae and a few principles. BTW, I specialise in Accounting/Finance.
However, it does get harder in 3rd year. There's a lot more to learn - and 3rd year finance subjects on investments (shares, bonds etc) are actually mathematically challenging. 3rd year accounting requires a fair bit of reading - I'd say as much as law subjects like contracts. But definitely not on par with constitutional law. But the thing is... if you are good at maths, you'll be fine with finance. If you're good with memory work, you'll get through accounting.
But with law, there's always more and more reading to be done. And the judgments are so bloody confusing...
You shouldn't be so worried about it. If you do get into Comm/Law, you'll get by... presumably if you got into it you're a high performer. Therefore, you should maintain pretty good standards (credits, distinctions). One thing to be aware of is that it's a lot harder to get 90s in uni. In fact, it's hardish to get 80s. So don't be too disappointed if you're grades fall back to 70s.
Also... Comm/Law isn't the be-all-and-end-all degree. Comm/Engineering is also looked upon very highly to the extent that if you get good marks, there's no difference as far as looking for a commerce job goes. I know a few Comm/Eng people who got into those IB or consultancy graduate positions...