I have a friend who is studying at USC so I know a little bit about the program. Probably the most important thing to say about USC is that it's a very small, tight knit cohort of ~30 people. You'll be with the same USC-Griffith provisionals for the entire 7 years (who you'll get to know very well) because even when you start Griffith MD you'll be doing it at Sunshine Coast Hospital so you don't mix with the rest of Griffith MD, is what I heard. Versus a bigger cohort at Melbourne. I don't know which is better.
Second is yes you are right Sunshine Coast is quite small, for example there is only one nightclub there so it comes nowhere near Melbourne in terms of city size, food options, entertainment options, etc. But it's around an hour drive from Brisbane so if money is not problem I reckon you could get a car and drive down to Brisbane during the weekends to get some good food if you really miss it lol. I heard a lot of people have cars up there.
Third is certainty of getting into medicine. I'm not an expert on Melbourne FFP but I've read a few explainers by
A1 and it doesn't seem like a very certain pathway to me. With USC you're guaranteed into MD, but ~6 out of the ~30 provisionals will be getting bonded medical place depending on their overall Med Sci GPA. Personally I think I'd be a bit stressed out about this but in the end you end up as a doctor nonetheless.