I would definitely put Commerce/eng above eng/law. Degree is 1 year less plus Commerce would be more helpful than Law, in terms of management, finance, economic feasibility and other stuff.
As a matter of fact, all engineering streams have to complete a compulsory management based subject in their final year (units called either Professional Practice or Engineers in Society).
Also Commerce is more science/maths based than Law is. Plus a Law degree isn't easy. It requires a fair amount of reading and massive essays to be written. Plus Law is competitive; your results are based on a bell curve instead of raw marks.
However, if you're really a maths/science student, then I would suggest doing sci/eng. You shouldn't go into uni doing just what is better for employment, you should go into it doing something you love. I plan to work as a Chemical engineer. My science degree is simply for interest because I like Maths quite a lot, plus it leaves the option of pursuing postgrad maths many years down the track.
Trust me, you'll enjoy the next 5-6 years of uni doing something you love rather than something that might help later on