That's a tough call on which is going to be cheaper, the Honda or the Caddy. My guess is that the Cadillac would be a little cheaper, possibly due to the fact that Honda Accords are among the most often stolen vehicle. If you don't put comprehensive (which is theft) on the Honda, then it will be a lot cheaper.
As for auto insurance, there's really three component's to it and here they are:
1. Liability - you have to have this. This is the coverage that will kick in if you hit another car and or injure another person. It is the law in every state to have this. I recommend going at least one level up from state minimum, it's not much more in premiums to get a lot more coverage. Like me for example, I have several times the minimum and it's only about $30 more every six months, so in my mind it's a good thing for me to have as I'm protected up to a much higher level.
2. Comprehensive: This is an optional coverage. If it's an older car that's not worth much money, you might not get it. This essentially covers you if the car is stolen. Also, it will cover you if your car is damaged by an accident that you did not cause like say a tree falls on your car.
3. Collision: This is likely one of the more expensive parts of your auto policy. This will help pay for repairs/replacement of your car if YOU cause an accident. If you are in an accident and it's clearly not your fault like if you are rear-ended, the other driver's insurance should pay. Collision is for if you cause the accident.
There's also other things like full glass coverage, underinsured/uninsured motorist coverage, etc. You need to decide if you need comprehensive and collision on your car. If you are making car payments, you will need all three. If not, it's your call.
As for how to insure it, talk with your parents about them being the owner of the car and listing you as an occasional driver. I did this when I was 16-17. I had a car, it was clearly for my use as I don't think my mother ever drove it and I think my dad may have driven it once. However it was in my parent's name on the title and I was listed as an occasional driver. This made the jump in insurance much less, and I paid them the difference. That might be your best bet.