I go to USYD and I visit UNSW every week. I went to a really asian school and the majority of my grade went to UNSW.
Two main things you should know. Generally first year the main ways you meet people is through knowing people already and then meeting new people through them, extra-curricular/society stuff and tutorials (lectures somewhat as well). If most of your cohort isn't at USYD and you're in an Arts faculty subject where you will only see the people in your tutes once a week and probably only for a semester, it's quite difficult meeting new people unless you're cray. That being said, if those don't apply to you social life at USYD imo is really good, lots of 'hubs' of where people hang (two bars, lawns, food hub, International student lounge, pool etc...), but those 'first' opportunities are harder if only 7 or so from your school are at the uni.
Another thing is that the guy somewhere above is kind of right- social life will be determined by your attitude in your first year BUT the uni environment will play a huge part; consider where most of your friends are going, the degree your doing (arts vs something smaller like law, architecture), activities you'll be doing etc. I've also heard that at UNSW you might know people but that might stop you meeting new people etc.
I've made a lot more friends visiting UNSW than I have at USYD but I think it's probably a as-time-goes-by thing. Wish you all the best in your decision making!
