Honestly, if I were in your position I'd still take the Monash offer purely due to the reasons you stated (eg: money, mentors, possible private spaces you can use etc)
You really need to think of the reasons why you'd prefer Melbourne over Monash. If its just the fact that it has" pretty buildings" and that to some Melbourne may appear "more prestigious", then they aren't good reasons to come Melbourne Uni.
For me (if I was an employer), looking at which uni you went to wouldn't be a major factor especially between such "prestigious unis" as you said. But more so about your marks and your extra-curricular activities. Even a student from deakin or la trobe would be consider over a student from melbourne or monash if they had far superior results.
Have a think, why did you choose Monash as your first preference in the first place?