Well, it kind of depends on what aspects of commerce you are interested in studying.
You said that you want to study commerce and also double in economics. I assume this means you want to study a commerce major like accounting, finance, marketing, etc. and also either major or do a bachelor degree in economics. If you wanted to go this route, I think Monash would be best because you can do a Bachelor of Commerce/Bachelor of Economics double degree.
Keep in mind that 'economics' is a possible major under the Bachelor of commerce degree at both Monash and Melbourne, so you could enroll in commerce at either university and major in economics and probably use your elective units to form another major in the commerce area if you wish (you can definitely do this at Monash, but I'm not too sure how it works at UoM with its breadth units). This has the bonus of only taking 3 years vs taking 4 years if you did a double degree.
As for what university provides the best outcomes for graduates, provided you get good marks your employment opportunities will most likely be pretty much the same.