Is a Ford Mustang a good car?
Mustangs and Focus are both good cars. Be careful with a used Mustang especially if it is a GT, these are usually driven hard, but if you are looking for just a basic Mustang then go for the V6 model.
If you are looking at a new Focus, they are a good car with great fuel economy and right now Ford is offering some pretty good deals on them. Since your grandfather works at a Ford dealer he could also get an employee discount.
Go with what would be cheaper for you to insure, maintain and fuel. Talk to your grandfather and see what he tells you, after all he knows you best and I'm sure he has a good idea of what would be a better car for you.