well i'm biased

but i genuinely think that Melbourne would be a better pick if you want to do actuarial.
1) You've mentioned the prestige of Melbourne, i'm generally not a huge fan of the melb/monash 'rivalry' but in this case there is a difference. The fact that the centre for actuarial studies has been around for a lot longer and is very well establish, and has a very favourable reputation. Monash was only recently accredited (only last year i believe) where as ALL of the all of the accredited subjects are offered and will get you part 1 and 2 exemptions if you do well enough.
2) Having a double degree can be good or bad, it depends what you want. It's good because you get a bit more of a breadth of experience, but having a two extra years of real work experience can be just as powerful, if not more powerful than having another degree, that's why i'm keen to get out and get some work experience and see what i like, i can always ( and i plan to) go back to uni one day to do a masters of something
3) Seeing as you'll have to be travelling for 5 years, travel is a very real factor, if Melbourne is closer, then it really will save you a lot of time over 5 years to choose the uni closer to you.
hope that helps!