Depends on:
where you live
what you want to venture into and why
whether you have scholarships or not
whether you want a double degree or not
But here are some general advice:
1) If you want to venture into heavily quantitative fields or do research, I'd suggest doing science and majoring in mathematics/statistics. Monash has a pretty rigorous financial and insurance maths program that you might want to consider. Doing any quantitative stuff with BCom is kinda limiting, unless if you're really good.
2) If you're looking to get a good job, both unis are already pretty competitive, and it's really up to the individual.
3) If you want to actually LEARN stuff then BCom may not be the BEST thing to do.
4) In general, the lecturers and the quality of the content taught in Melbourne is better than Monash (and I say this as a Monash student who have looked into the content of both unis), but Monash has a bigger advantage when it comes to Econometrics and Business Analytics, because all the good researchers are in Monash, otherwise Melbourne takes the W here.
5) Uni (especially BCom) depends on what you make out of it. Don't see why prestige matters in the workplace unless if it's academia.