Not at Monash, but here is what I think:
If you have a natural inclination towards questioning assumptions, particularly political assumptions, you will probably get frustrated with international studies at some point. This is pretty much what happened to me. I took political science in my first year of university (there is a lot of overlap between political science and international studies) and found it dreadful, it only really reinforced dominant views of neo-liberalism or was outright Marxist. I wrote in one of my essays something to the effect of 'if you assume that democracy is a desirable goal, then X follows'. Written in red on my essay was 'in political science, we don't make assumptions.' Which, isn't true at all. I mean, the existence of the nation-state is an assumption (granted, they do question it, but in the end they still take it for granted) you could even argue that our very existence is an assumption. In life, you have to assume a lot of things and in political science you evaluate political systems by assuming certain values, you can justify the assumption but that doesn't mean you have some claim to truth. That's life. This doesn't mean that you shouldn't do it, but it is something to consider.
On the other hand, if you have problems with thinking about assumptions too much (to the extent that you aren't sure whether you exist and that depresses/worries you), then philosophy is not the way to go. There is a reason why all of those existentialists killed themselves and it probably only in part has to do with mental illness. Philosophy can be quite emotionally difficult at times unless you can effectively compartmentalise it. Additionally, if you think questioning assumptions is absurd (for instance, if you think asking 'why is a pen a pen?' frustrating, weird and pointless) then you also probably don't belong in a philosophy class.
In essence, I would base the answer on your disposition. Both are important disciplines and both can be very interesting.