Maybe Orlando has one of the busiest ERs in the country, but Ryder Trauma Center (a free-standing trauma center in addition to the regular ER) is one of the best and busiest in the world. Not saying that UCF doesn't have something to offer here, but if trauma's your thing, not much will compare with UM.
I'm a student at UM (happy to answer any questions you have), so I'll try extra hard to not be biased but I don't know much about UCF. I think there are 3 general things that go into making a decision like this. Do you feel like you fit at the school? Can the school help you go where you want for residency? What will each program cost you?
1. Since you're still considering both after your acceptances, obviously you feel like you fit at both.
2. You say you're interested in rad onc (but good for you for being open to other specialties!): Sylvester Cancer Center is a great hospital. Since the JMH/UMH hospital system services all of South Florida and the Carribean, you'll see a ton of patients here, with all stages and types of cancers. We have a ton of research here too, with some leaders in the field. These are experiences that make your residency app look just that much better. I really don't know much about UCF's cancer stuff, but I'm sure it's good too.
3. UM is private (though the second cheapest private med school in the country, and we offer a ton of scholarships) while UCF is public. For in-state, UCF is probably going to be cheaper.
As a student, I can give you one other piece of advice that many will disagree with me on: don't go to a school because your family is there (with perhaps the exception of a SO). I wanted so badly to go to school near to where my parents were, but am SO HAPPY that I didn't. Rad onc is a really competitive specialty: you'll need to work hard in med school. Living near to family is an additional pressure to spend time with them. Were I nearer to home, I would feel guilty about not spending as much time with them. People outside of med school rarely realize how much time this is: between academics, research and other extra-curriculars, I fill up probably 14/15 hours of my day, 7 days of the week (I also want a competitive specialty). Of course I miss my parents, but now when I see them, I don't have the stress of an upcoming test or a project deadline poisoning my time with them.
Best of luck with your choice (but Go 'Canes!)