The only med interview I've done was the UWS one last year to get in this year and I got in, so I will never know which is more difficult. Sorry.
To prepare I just had a friend of mine talk to me on the phone the night before to get me talking and also she asked me a bunch of ethical questions she remembered from when she helped her other friends try for USyd med.
Okay, coolios. Personally I wanted to go to UWS more than anywhere else (shorter course and the campus is 10 minutes drive from my house), but I wouldn't have gotten into UNSW anyways.
Good luck